Design Analysis of
The North American B-25 Mitchell

By C J HANSEN,
Chief Project Engineer, North American Aviation, Inc.

This distinctive gun-bristling medium bomber — first warbird to drop "eggs" on Japan — has again and again played a strikingly effective role in far-flung war performances. Here's a meaty presentation of basic B-25 makeup features and comparative details of models H and J … The 11th in Aviation's peerless design analysis series.

From the outset of the war, the North American B-25 Mitchell medium bomber series has earned an enviable reputation in the hands of United Nations pilots in all theaters of combat. Designed in 1939 and first flown in 1940, this plane has been developed through a series of changes brought along by practical needs.

Latest models of the Mitchell to enter service are the B-25H — outstanding among production planes for its extremely heavy armament — and the B-25J — latest improvement on the original medium bomber design.

These are pointed examples of North American's design policy of ease and economy of fabrication. In this instance, the Mitchell was manufactured on the basis of 48 major assemblies, any one of which, theoretically, could be modified without necessarily affecting the other 47. The B-25 has undergone nearly a dozen such changes without altering the basic design.

In both models, firepower has been increased by addition of a tail turret, waist guns, moving the upper turret forward to improve field of fire, and two fixed forward-firing package guns on each side of pilot's compartment.

Nose of the B-25H is fitted with four machine guns and a 75-mm cannons; the B-25J with two machine guns — one flexible and one fixed — protruding from the bombardier's enclosure.

Tactical purpose of the B-25H is primarily for low level attack and the destruction of land or naval materiel targets, in support of ground, air, and naval forces. As a dual purpose plane, it can knock out surface forces protecting the target, then proceed to bomb the target from low altitudes. The B-25J is primarily a medium bomber, nevertheless, it is capable of dealing with surface forces.

In spite of their versatility, current B-25's virtually remain unchanged from the original. They are semi-monocoque mid-wing monoplanes powered with two Wright 1700-hp 14-cyl engines. All fixed surfaces are metal-covered. Flight control surfaces, exclusive of wing flaps, are heavily doped-fabric covered members. Countersunk rivets are used on the forward one third of fuselage skin and fixed surfaces to reduce drag, and brazier head rivets on the aft two-thirds.

Normal entrances and exits consist of two hatches in the plane's belly, one under the upper turret compartment, the other aft of the waist gunner's position. Automatically retracting stepladders are provided.

Escape hatches are for use in crash landings when main entrance hatches are blocked, or under emergency conditions while the airplane is in flight.

Crew for the B-25H consists of a pilot, upper turret gunner, cannoneer-navigator, waist gunner and tail gunner. Pilot, co-pilot-navigator, bombardier, upper turret gunner, waist gunner, and tail gunner man the B-25J.

Both bombers now carry considerably more armor plate to afford maximum protection for the crew and vital equipment. Provided also is a rubber lifeboat, emergency equipment, pyrotechnics, and the usual oxygen equipment.

Center Section Structure

Backbone of the B-25 is the bomb bay section, built integrally with the wing center section. To resist a major portion of fuselage bending loads carried by this section, 24ST H-shaped extruded lower longerons and Z-shaped upper longerons are employed. Fore and aft fuselage frames, to which longerons attach, are composed of 24ST webs riveted to right-angle alloy extrusions. These frames are attached to front and rear wing spars and serve to introduce flight loads into the fuselage.

Intermediate similarly constructed frames serve as attaching members for the bomb rack structure which is stressed to carry part of the fuselage loads. Top fuselage frames in this section support bomb hoisting loads.

Sides of the bomb bay compartment are additionally supported by intermediate frames, plate ribs, rolled Alclad stringers, and channels. Bomb rack rails which support bomb loads are bolted to bomb rack attachment frames. Roof of this compartment also serves as the floor of a crawlway which permits crew travel fore and aft during flight. This floor is .032 24ST Alclad stiffened by hat-shaped rolled angles and extrusions. Transverse shear loads are borne by the floor along with the top fuselage skin. At this section the fuselage is skinned with lap-jointed 24ST Alclad varying from .032 on top to .051 on the sides.

Bomb bay doors have inner and outer skins. Inner skin is a single panel lightened by circular cutouts, attached to a framework of stringers and frames. Outer skin is riveted to and supported by the same framework. Doors are hinged to the adjoining structure at three points.

Wing Center Section

The wing center section is attached to and supported by the intermediate fuselage section or bomb bay compartment. Two main spars, front and rear, serve to resist spanwise bending loads and extend 157 in on either side of fuselage centerline.

Spars consist of 24ST aluminum alloy webs, .081, .064, and.051, to which extruded capstrips are riveted. A firewall of .019 stainless steel acts as a doubler and extends along the forward side of the front spar behind the engines. Extruded stiffeners bolted and riveted between the capstrips strengthen the webs.

Landing gear beams between front and rear spars begin to a point 97 in from the fuselage centerline, and consist of .051 24ST Alclad webs and vertical 24ST extruded stiffeners. Attached to the lower part of landing gear beams are gear lock fittings and vertical brace fittings. Vertical load at these fittings is applied to the web by means of adjacent vertical stiffeners. Purpose of the landing gear beam is to resist vertical loads when landing and while the plane is supported on the ground by means of the landing gear.

An intermediate beam, known as the gas web, is located between front and rear spars and extends outward from the fuselage for a distance of 97 in on each side. This member separates the two main fuel compartments and carries some of the flight loads. It consists of an .081 24ST Alclad web and 24ST hat-shaped stiffeners riveted at 10-in intervals.

Six center section ribs distribute a major portion of landing gear fitting, landing gear beam, and engine mount fitting loads. Wing joint plate-type ribs at each end of the center section serve to redistribute the normal shear and torsional moment from outer panel to center section. They are formed principally of .064 24ST Alclad, and are supported vertically by stiffeners. Similar ribs placed 97 and 133 in from either side of fuselage centerline are made, respectively, of .064 and .091 24ST Alclad and aid in distributing landing gear and engine mount fitting loads.

Upper center section surface is skinned with 24ST Alclad from .032 to .081 thick, riveted to ribs and transverse stiffeners, butt-jointed spanwise and lap-jointed chordwise. Lower center section surface is fitted with .051 and .064 24ST panels, removable to provide access to fuel cells located in the wing. Panels are stiffened chordwise by hat-sectioned formers and by spanwise stringers. Fuel tank securing fittings are attached to inner surface of the panels.

Center section leading edge is conventionally constructed of rolled 24ST Alclad skin stiffened by stringers and supported by formed nose ribs. Inboard flaps constitute the center section trailing edge.

Fuselage Forward Section

Forward fuselage section extends aft from the nose to the bulkhead at front end of the bomb bay. This section consists of the hinged hood and fixed lower "front" section on the B-25H, or bombardier's compartment on the B-25J, and pilot and upper turret compartments on both models. Main bulkhead for support of nose gear fittings and pilot's compartment armor plate fittings, is located at rear of front section.

Pilot's cockpit enclosure is made of Lucite or Plexiglas, and glass sheet, set in metal frames and sliding panels. Floor section is an integral structural feature of the fuselage. The 75-mm cannon on the B-25H is in the crawlway used by the bombardier on the B-25J. The cannon housing is on the left side of the pilot's compartment under the cockpit floor, and serves, structurally, as a torque-box.

Cannoneer's compartment on the B-25H is known as the upper turret compartment on the B-25J, and is located behind the pilot's enclosure. It includes the front entrance hatch, upper turret guns, and on the B-25H is used as the loading station for the cannon.

Four longerons are the largest single load-carrying fuselage members, and bear axial tension and compression forces due to action of bending. Shape and size of each depends on the load carried, and except for the extruded longerons in the bomb bay assembly, they consist of channels formed from 24SO Alclad sheet later heat treated to 24ST.

Two lower longerons extend the full length of the fuselage. They are continuous within individual fuselage sections later bolted together to form a running fuselage structure. In the fixed portion of the front forward section, the left longeron (on B-25H and J) is a Z-shaped member formed of .051 Alclad (later heat treated). Because of higher load factors, thickness of the corresponding right lower longeron is increased to .091.

From the front forward section extending back through pilot's compartment to front bulkhead of the bomb bay section, lower longerons are downward-flanged .064 24ST Alclad channel members, reinforced along the skin side flange by extruded angle. In approximately the middle of pilot's compartment an .081 rear section is spliced in with a .064 doubler of the same material.

True upper longerons in the forward section are located only between the bomb bay and pilot's compartment rear bulkhead and are .091 24SO formed channels heat treated to 24ST. They are otherwise replaced by members known as bombardier's rails and pilot's rails, and serve as upper extensions of the longerons. "Breaking" of the upper longerons and continuation of the structural frame on a lower plane is made necessary by the cutout for the glass enclosure in pilot's compartment.

Each pilot's rail runs longitudinally along the side of the fuselage, just below pilot's side window. Rails are attached to the fuselage finishing angle at the end of the gun turret compartment, extend forward, and are connected to the front bulkhead of pilot's compartment. Each is a downward-flanged L-shape channel member of .091 24SO Alclad sheet (later heat treated) to which an extruded angle is riveted. Forward and aft of a bulkhead located approximately at the first frame of pilot's enclosure, loads diminish in intensity and the downward flange of the extruded angle is machine tapered.

Pilot's compartment glass enclosure frame is secured by wing channel nuts riveted to the inner flange of pilot's rail.

Second forward extension of upper longerons is provided by bombardier's rails which extend on each side of the fuselage from a bulkhead in line with first pilot's enclosure frame to the front of the forward front nose section on the B-25H.

On the B-25J, these rails constitute the base of bombardier's transparent enclosure, serve as the main structural members of the nose section, and are on a lower plane than pilot's rails. From the front bulkhead of pilot's compartment forward, they are downward-flanged .051 24SO Alclad channel members (heat treated to 24ST after forming) and are .064 thick, rearward. Except for armor plate, fuselage is skinned with 24ST Alclad ranging from .051 to .025.

Rear fuselage section includes the portion aft of the bomb bay compartment. Upper longerons are flanged channel sections of formed and subsequently heat treated 24SO Alclad sheet, diminishing in thickness from .091 to .051 at the tail.

Lower longerons are broken at the waist gun position and consist of two U-channels of .081 24ST riveted together to form an H-section. They are reinforced along the inside flanges by a .091 24ST Alclad doubler riveted to the bottom inboard side of the lower U-section for a distance of 12 in. immediately aft of the bomb bay rear bulkhead. Lower longerons aft of the waist gun positions are channels formed of sheet Alclad of diminishing thickness.

Rear fuselage skin is 24ST varying from .064 to .032, depending on stresses involved, and is riveted to fuselage stringers and formers in the usual manner.

Fuselage frames are formed from sheet Alclad varying from .032 to .064, depending on local stress or use to which they are to be put as supports for attachment of miscellaneous flight equipment. Except for a few frames located at points of particular stress or those used in attachment of the various fuselage sections, they are open channels or angles provided with stringer cutouts, and in some cases with holes for passage of cables, conduits or plumbing lines.

Specially stressed frames, or bulkheads, are located at the end of the front forward section where the nose landing gear fittings are attached: two frames in the bomb bay compartment where bomb racks are installed; two at points of attachment of horizontal stabilizer to fuselage; and two which carry lift loads from the wing into the fuselage sides at the point where front and rear wing spars pass through the center section. These frames are generally a web of sheet metal reinforced by extruded capstrips, riveted around the web perimeter.

Stringers used in the B-25H and J fuselages are rolled sections of 24ST Alclad except for a few extruded shapes of 24ST.

Fixed Surfaces

Each outer wing panel is assembled about a main spar located at the 33 percent chord line. Secondary false spars extend the length of the panel along its trailing edge. Remaining structure consists of ribs pressed from 24SO and 24ST Alclad sheet, extruded spanwise stringers and 24ST Alclad skin. Access and inspection doors are provided along the entire wing, most being on the lower skin.

Outer main wing spar consists of inboard and outboard sections of 24ST sheet spliced with a .064 Alclad doubler plate. Inboard spar section is .081 material formed with upper and lower U-shaped flanges serving as spar caps. The web is stiffened by vertical rib attaching right-angle extrusions and independent stiffener angles.

Outboard of the wing center section attaching plates, the web is cut away to permit installation of two oil cooling ducts. Each cutout is reinforced by a T-section extruded ring riveted around the cutout. Inboard spar section is additionally reinforced by an .081 24ST Alclad doubler extending between the ribs on either side of he cutout. The outboard flanged web is .064 Alclad with upper and lower edges formed to provide spar caps. Eight flanged lightening holes are provided in the spar tip.

The outer wing flap false spar is .040 24ST Alclad with formed right-angle flanges for spar caps. Between the endplate and the third rib, the spar of a single sheet of .040 24ST section riveted to upper and lower spar caps. Inboard web section has several cutouts to accommodate equipment.

Ailerons are supported by a false spar of a single sheet of .040 24ST Alclad. Formed right-angle flanges on the upper and lower edges provide spar caps. The web is stiffened by several rib attaching angles of extruded alloy arranged vertically along the solid web.

Majority of the wing ribs are pressed from 24ST or 24 SO Alclad varying from .025 to .040. All ribs forward of the main spar are provided with flanges riveted to the skin. Most intermediate ribs between main spar and false spars are formed so they do not contact the skin but are attached to stringers by means of clips. All ribs are provided with conventional lightening holes and stiffening beads.

Wingtips are constructed of three ribs and several formers, all press-formed from Alclad sheet stock. Beaded lightening holes, stringer cutouts, and flanges are provided in each member. Contour skin is formed from 24SO and tips are covered with 24ST Alclad and attached to the wing with screws.

Horizontal stabilizer is a full cantilever stressed skin structure of pressed ribs, spanwise stiffeners, and two spars. Front stabilizer spar is a web of .051 24ST Alclad spliced to a heavier web of .064 material where it covers the fuselage. Beaded lightening holes and angle vertical members stiffen the web. Top and bottom capstrips are extruded angles riveted into place. Rear spar is of substantially the same construction except that the web is .051 throughout.

All ribs are made of .032 24ST Alclad with flanges acting as caps and stiffening beads pressed in forming. Lightening holes and stringer cutouts are also provided. Stringers are extruded 24ST bulb angles and formed J-sections. Stabilizer skin is .025 and .032 24ST Alclad riveted to the framework.

Vertical stabilizers are similar in structure and are attached to the horizontal stabilizer with standard AN bolts at the junction point of front and rear spars.

Movable Surfaces

Wing flaps of the B-25H and J are the trailing edge slotted type, consisting of two sections on each side of the fuselage. Inboard flaps extend from the nacelle tail cone to the side of the fuselage, and outboard flaps extend from the aileron to the nacelle cone. When in neutral position, flaps are sealed by a non-metallic strip attached to the upper wing surface, and large slot openings on the lower surface of the wing are closed by small fairing doors.

When flaps are moved to a down position, fairing doors swing upward permitting flow of air up between the fixed wing and the lowered flap. Rubbing strips of .010 half-hard steel sheet are provided along the flap leading edge at two places. Actuating torque of each flap is taken by a torque tube which extends into its interior. Flaps are supported by, but not hinged to the torque tube.

Flaps have power-pressed ribs and spanwise stiffeners. Ribs are provided with stringer cutouts and lightening holes. Trailing edge is formed of .032 24ST Alclad.

Sealed type ailerons have a front spar and pressed ribs. Conventional weights are attached to the leading edge to obtain static and dynamic balance.

Aileron main spar is 137 in and is formed from .032 24ST Alclad sheet into a U-shaped channel, joggled on the outside upper and lower cap edges to provide for attachment of fabric replacement strips. Rib attaching angles are placed vertically on the spar web. Lightening holes are spaced along its entire length. A light false spar is provided as a trim tab attaching member. Ribs are formed from .032 24ST Alclad and include formed top and bottom frames and beaded lightening holes.

A trim tab, mounted in the trailing edge of each aileron, consists of a small metal airfoil, of rectangular plan, having a U-shaped spar, triangular shaped ribs, and an Alclad covering. Hinged at three points to the aileron false spar, the tabs serve as control boosters in addition to trim units and are not balanced.

Both elevator and rudder spars are formed of .040 24ST Alclad with 7/8-in flanges along top and bottom surfaces. Flanged holes lighten the structure and stiffen the spar web. Rudder and elevator false spars are U-shaped, formed from .032 24ST Alclad. Trailing edge ribs attach to these members, and brackets are used to attach trim tabs at three points along the flat surface of the spar. Nose skins for both members are rolled from .025 24ST Alclad and are riveted to main spar flanges and rib caps. Rudder nose skins are provided with flanged lightening holes. Rudder and elevator trailing edges are U-shaped members of .025 Alclad riveted to the rib tips. Trim tabs are constructed of sheet metal in the conventional manner.

Fabric covering of the elevators and rudders is grade A mercerized cotton, doped and finished. Covering is attached to dimpled holes in the trailing edge rib capstrips by countersunk sheet metal screws inserted through dimpled washers. Doped reinforcing tape is placed along the ribs before the screws are inserted. After insertion of the screws and washers, a strip of finishing tape is placed over them to provide a smooth surface.

Nacelles and Engine Mount

Engine nacelles are broken down into three main divisions — engine cowling section, front section, and rear section. Engine cowling section consists of a nose ring, cowl formers, and cowl panels. Material is aluminum alloy and corrosion resistant steel. The entire structure is supported by the front section frames bolted to the bottom of the wing center section.

Nacelle front and rear longerons are rolled Z-section members .091 and .064 24SO Alclad, respectively, heat treated to ST condition after forming. Frames are typical pressed members of 24ST Alclad of varying thickness and have right-angle flanges and stiffening beads for added strength.

The firewall web is made of stainless steel sheet, spotwelded to a series of stiffening members of varying size and cross section. Holes are provided to accommodate electrical and hydraulic lines.

The engine mount is X4130 chrome molybdenum steel tubing, arc welded into a single unit, bolted to the wing structure at four points. The engine is attached to the mounting ring by cushioned dynamic fittings. Tube diameters range from 1¼ to 2 in OD with wall thickness varying from .049 to .094.

The B-25 was the first production medium bomber type to incorporate the now widely used tricycle landing gear.

Main landing gear is of the single-leg, half-fork, full-cantilever design and is fully retractable into the engine nacelles. Each shock strut consists of a cylinder and piston, shock being absorbed by air and oil. Piston and cylinder are interconnected by forged chrome molybdenum steel torsion links which transmit torsional moments from piston to cylinder. Torsional loads applied during landing are transmitted through the shock strut to the side thrust brace, two trunnions, the downlock and the main supports.

A 47-in tire is mounted on each of two cast aluminum alloy drop-center-type wheels carried on hardened steel axles over which are pinned chrome molybdenum steel forks. All chrome molybdenum steel landing gear parts are heat treated to a tensile strength of 160,000 – 180,000 lb/sq in. Each wheel of main landing gear is equipped with hydraulically operated, multiple-disk metal-to-metal clutch type brakes.

Nose landing gear consists of an air-oil shock strut and 30-in wheel. Like the main landing gear, the nose gear is fully retractable. In retracted position it rests in the fuselage underneath and to the right of the pilot. All torsion and bending loads are transmitted and reacted in the same manner as they are in the main gear. The nose wheel has a steering range of 60° left or right, allowing a turn with either main gear as a pivot point. A hydraulic shimmy damper is mounted on the strut which also incorporates a self-centering device to center the wheel when the weight of the plane is removed from the nose gear.

Hydraulic System

The hydraulic system — single-pressure type — operates the tricycle landing gear, wing flaps, cowl flaps, bomb bay doors, and brakes. Cowl flaps have separate control handles for left and right engines, and operation of either the cowl or wing flap may be stopped at any desired position. A manually operated emergency hydraulic system permits actuation of the different sub-systems should both engine-driven pumps fail, or when the airplane is on the ground with engines not operating.

The Pesco 349P hydraulic pumps are engine-driven, two-gear, positive displacement types with a pressure limit of 1,500 lb/sq in for continuous operation, will operate in either direction, and are completely lubricated by the fluid passing through. They are located on the engine accessory sections and either is capable of providing sufficient pressure for the system.

Fluid from the reservoir is forced from the pumps through lines routed back of the forward wall of the bomb bay where the lines from both pumps join. Check valves are provided so that failure of one pump will not render ineffective the pressure produced by the operating pump. The pressure line continues on from a T-fitting through a check valve and then through all the system lines leading to the selective operating systems.

Fuel System

An independent fuel system is provided for each engine. Main source of fuel supply is from four self-sealing tanks, two located in each wing center section between the fuselage and the engine nacelle. Front and rear tanks on each side are connected by a line from the rear tank to an adapter to which a booster pump is attached. A check valve permits fuel to flow from the rear to the front tank and then to the engine, but prevents fuel from returning to the rear tank.

Booster pumps are operated by switches on pilot's control pedestal switch panel. An auxiliary fuel supply is also obtained from six small self-sealing tanks, three in each wing center section outboard of the main tanks. Front and outboard tanks in each group are connected with the rear auxiliary tank by a line from each tank aft through the nacelle to an electrically operated transfer pump. The transfer pump draws fuel simultaneously from the three auxiliary tanks, pumps it through a line to the front main tanks.

Each engine is fitted with type G-9 rotary vane positive displacement type fuel pump located on the right-hand aft end of the engine.

Oil System

Each engine has an independent oil system, identical except for minor variations in oil line routings. A self-sealing tank is installed in the wing section aft of the engine firewall. Oil flows from the tank to the engine-driven oil pump, to the engine under pressure of 80-90 lb/sq in.

System oil is utilized for feathering the propellers and is taken from the circulating oil by a pump, mounted on the front of the engine. Temperature of the oil returning from the engine is regulated by two thermostatically controlled type C-8 oil temperature regulators mounted in the center portion of ducts which open in the leading edge of the wing and have outlets through the upper surface of the wing just forward of the wing flaps.

Surface Control System

A duplicate cable control system is installed to actuate rudders, elevators, and ailerons. Each system is so designed that loss, through gunfire, of any one cable will not seriously cripple the plane. Cables are color-banded to facilitate assembly, repair, and inspection of the control system.

Control forces originated by the pilot are applied to the control column, located on the left side of the compartment, connected to a steel torque tube extending across the fuselage and having take-off horns at each end. Elevator cables extend aft along each side of the fuselage from control column horns to bellcranks in the aft fuselage. Adjustable push-pull rods connect each bellcrank to the respective elevator horns.

Elevators are joined by a torque tube connected to each elevator horn, and may be raised 25° or lowered 10° with respect to the horizontal stabilizer. A bungee is incorporated in the elevator control system to reduce forces on the control column.

Ailerons are controlled by clockwise and counter-clockwise movement of pilot's control wheel. Cables are led out of the control column torque tube, then aft to the aileron sector on the rear wing spar. Maximum upward aileron movement is 28°, downward is 14°.

Rudders are actuated by hanging type pedals. Control cables extend aft along each side of the fuselage from the lower outboard ends of the rudder pedal assembly to the horizontal stabilizer, then outboard on each side of the stabilizer to rudder sheaves at the outboard ends of the stabilizer. Rudders can be moved 20° right or left with respect to vertical stabilizers. Loss of cables on one side of the airplane will not affect rudder control on the opposite side.

Elevator trim tabs are operated by control wheel on the left side of pilot's control pedestal. Aileron trim tabs are actuated by the forward control knob on the floor of pilot's compartment, and the aft control knob on pilot's floor operates the rudder tabs.

Power Plant and Propeller

B-25Hs and Js are powered by two Wright 14-cyl twin-row R-2600 engines, each rated at 1,700 hp for takeoff. The exhaust system uses Clayton-type flame-damping individual stacks which protrude through openings in the cowling. The Jack & Heinz JH-5F starter may be used as a conventional inertia starter to directly crank a warm engine without energizing, or to direct-crank after energizing, if desired.

Ignition is provided by two Scintilla SF14L-3 magnetos with 14-lobe breaker cams turning at half crankshaft speed. Both magnetos are timed to No 1 cylinder, and each is cooled by air from blast tubes running forward to the front of the engine.

Each engine is equipped with Model 1685-HA Holley carburetor to which flow of air is directed by a scoop mounted to the intake by a flexible joint. Main body of the air scoop is a casting containing a cold air door interconnected by linkage and controlled from pilot's control pedestal. A removable cover permits installation of either filter or baffle. When the cockpit control is in the Normal position, cold air is admitted to the carburetor, and when control is in Icing, warm, unrammed air enters from the engine compartment.

Power plants are equipped with three-bladed Hamilton Standard Hydromatic, full-feathering, constant-speed propellers, diameter 12 ft 7 in. Blade settings range from low pitch of 22°. to high of 90°.

Electrical System

B-25 H and J electrical systems are 24 VDC single-wire type with the aircraft structure serving as a common ground except where dual wiring is required to prevent compass deflection. Nearly all of the wiring is open type supported by clips and protected, where necessary, by insulating tubing, tape, or cord. Conduits enclose wiring in engine nacelles and wherever additional mechanical support or electrostatic shielding is necessary.

A 24 V 34-amp hr battery is located in each of the engine nacelles, aft of the firewall. Either battery will operate the electrical system, including starters.

Two engine-driven 200-amp 30 V generators, one mounted on the supercharger housing of each engine, power the electrical system. Each generator is cooled by a blast tube leading to the generator from just aft of the propeller disk.

Radio Equipment

Radio system consists of the command set used for plane-to-plane communication; radio compass receiver; multiplace interphone system; and emergency transmitter for use in the life raft should the crew be forced down at sea. The B-25J has a liaison set, for long distance plane-to-plane operation, as additional equipment.

The command set includes a rack with two transmitters, rack with three receivers, combined dynamotor power supply and modulator, antenna relay, and necessary command radio wiring. With exception of control boxes, the command set is located in the upper left forward portion of cannoneer's compartment on the B-25H, and in upper turret gunner's compartment on model J.

Radio compass consists of a receiver, remote control box, azimuth indicator, rotatable loop, and terminal junction box. The receiver unit is a 15-tube superheterodyne with a wide frequency range and is located on the forward portion of the cockpit floor, on the right side of the plane.

Emergency transmitter, consisting of a portable unit with self-contained generator, 300-ft antenna, kite, balloon, signal lamp, hydrogen generator, and parachute attached to two canvas bags in which the equipment is stowed, is used by personnel forced down in the water, and is pre-tuned to the international distress frequency of 500 kc.

Interphone equipment includes an amplifier, one jack box for each of the seven stations, and throat microphone and headset for each crew member.

Liaison set consists of a receiver and transmitter.

Gunnery Equipment

The B-25H has fourteen .50-cal machine guns and one 75-mm cannon, and the B-25J mounts twelve .50-cal guns.

The cannon assembly in the B-25H, consisting of type T-13 weapon mounted on type T-13E2 recoil mount, is situated in the tunnel beneath the left side of the pilot's compartment. Cannon muzzle projects forward through a blast tube in the lower nose section and the breech extends aft to the left forward side of cannoneer's compartment. Cannoneer loads the gun and pilot fires it.

The all-metal nose forms a compartment for the four nose guns and ammunition boxes. Upper portion of the nose is hinged to provide access to the guns and ammunition belts. Guns are mounted side by side, are charged by cannoneer and fired by pilot.

Two guns are installed on each side of the fuselage just outside cannoneer's compartment. Attached to metal brackets, they are enclosed by metal blisters fastened to fuselage. Ammunition boxes are supported on a shelf structure at each side of cannoneer's compartment. Blister guns are also fired by pilot.

The upper turret is installed on a support pedestal in the aft portion of cannoneer's compartment. The field of fire of the two .50-cal guns in azimuth and elevation is automatically controlled by cams and switches in series, allowing gunner to freely follow a target without bullets striking any part of the plane.

Waist guns are flexible, mounted on a trunnion yoke, and project through a canvas boot in a transparent glass blister, one on each side of fuselage aft of the bomb bay. Each gun is provided with bungee cables to aid gunner in maneuvering during firing periods. Ammunition is stored on a shelf aft of the guns and is led to them through fixed-feed and flexible chutes.

Tail installation consists of the Bell Type M-7 turret with a twin-gun adapter mounting two Type M2 .50-cal machine guns. Feed chutes are equipped with booster motors.

In the B-25J, the four nose guns and cannon are replaced by a fixed forward-firing .50-cal gun and one flexible .50-cal gun. The flexible gun, operated by the bombardier, is placed in a ball-and-socket mount installed in the foremost point of the nose directly above the bomb-sighting window. The fixed nose gun is mounted on the right side of bombardier's compartment, it's barrel projecting through the nose, and is fired by the pilot. Armament of the B-25 is otherwise identical with that of the B-25H model.

Bombing Equipment

The bomb bay is equipped with fixed ladder type racks designed to accommodate 100- to 1,600-lb bombs. A special rack may be installed to carry one 2,000-lb bomb, or another special rack installed to accommodate a torpedo. Controls enable pilot to release bombs from the racks either electrically or mechanically. A manhole is provided for ease in stringing bomb hoisting cables and permits inspection of the bomb bay during flight.

Oxygen Equipment

Two portable low-pressure oxygen units are stowed at the rear entrance of the bomb bay crawlway. When in use, one is mounted behind pilot's seat and the other is attached to the forward corner of cannoneer's compartment ceiling.

Type AN-R-5 demand regulator is mounted on the side of each oxygen cylinder and automatically controls flow and dilution of oxygen. As the user inhales, a diaphragm is inflated, opening a valve permitting oxygen to flow through the regulator. Oxygen then mixes with free air in an amount governed by an aneroid valve which controls an air port and an oxygen port. At sea level the air port is open and the oxygen port is closed. As the altitude increases, the aneroid expands, closing the air port until finally, at about 30,000 feet, the air port is completely closed and the regulator is delivering pure oxygen.

Instrument and Photographic Equipment

Instruments on the B-25 are divided into four general classifications: Vacuum system, airspeed system, engine system and miscellaneous. Normal complement of instruments applicable to a twin-engine bombardment plane is utilized and offers no unusual installations.

A type K-24 camera is located just aft of the bomb bay in the fuselage rear section. Photographs are taken through a window in the fuselage floor, through a range of 50° fore and aft of vertical.

Cold Weather Provisions

Three interchangeable Stewart-Warner fuel-air heaters are used, each with an output of 50,000 Btu/hr. Ventilating and combustion air is supplied by ram from openings in the cannon tunnel, wing leading edge, and external air scoops in the fuselage.

The forward heater is located aft of the nose guns and supplies hot air for defrosting transparent areas of pilot's enclosure, and heat for pilots and navigator.

Second heater extends into the left center section wing and supplies heat for cannoneer's compartment and upper turret.

The rear heater is mounted aft of the left waist gun window and furnishes hot air for defrosting waist and tail gun windows. Provisions have been made for use of electric gun-heaters when needed.

This article was originally published in the March, 1945, issue of Aviation magazine, vol 44, no 3, pp 119-142.
The original article includes 4 photos, a three-view, 29 detail drawing/diagrams, and 6 data tables. Photos are not credited.
A PDF of this article [PDF, 8.7MiB] is available on the site.