The Boeing B-17 Flying Fortress

By Wellwood E Beall
Vice-President in charge of Engineering, Boeing Aircraft Company

Renowned pioneer of four-engine heavy bombers, this fast long-range aerial battleship has won unstinted praise of pilots and crews and wholesome respect of foe in every war-theater. This 10th in Aviation's unparalleled series of design analyses presents a comprehensive word-and-illustration treatment of B-17 basic details of structure and equipment that underlie the bomber-slugger's reputation.

The Boeing B-17 Flying Fortress was designed as a battleship of the air — a stable sturdy platform from which to drop bombs. It is daily living up to its picturesque name and designed purpose.

From every fighting front of the first global war in history come reports confirming that when this first four-engine, heavy bomber was conceived, a third major striking arm — airpower — was added to the basic forces of war. Just as artillery gave to land armies a weapon of concentrated long-range force, and the battleship did the same for sea power, so has the heavy long-range bomber become the core around which airpower is fashioned.

In 1934, Army Air Corps requests for bids on a "multi-engine bomber that would fly 250 mph, land and stop within 2,000 ft, over a 50 ft barrier, was interpreted by Boeing officials to mean two or more engines, and on that assumption, the four-engine design of the Fortress was based.

In addition to its flying and fighting characteristics, it was conceived by Boeing officials that the plane would have to be designed to permit manufacture in large quantities, provide for interchangeability of parts and service and repair under adverse wartime conditions, and be sufficiently conservative in design to allow for modification in keeping with the ever-changing conditions of war. The rapidity with which the Fortress has changed is convincing proof of the wisdom of their early decision, but despite thousands of changes incorporated in the several modifications of the plans, the basic design and most of the major specifications still remain.

The B-17's structure is basically one in which the skin and stiffening members, such as corrugated sheet, I-beams, angles, tubes and rings of formed sheet carry the loads. Except for the engine mounts, firewalls, landing gear, two tubes in each of the forward and aft walls of the bomb bay, and other miscellaneous alloy steel fittings, the airplane is structurally constituted of 24ST and 24ST Alclad.

The wing has a span of 103 ft 9.38 in — same as the original Fortress. The airfoil section combines NACA 0018 at the root with NACA 0010 at the tip. Wing area is 1,420 sq ft and the root chord is 228 in. Tip chord is 106.7 in. Taper ratio is 2.34:1, incidence 3½°, dihedral 4½°, and sweepback of the leading edge is 8-1/4°. In taxi position the wing chord angle to the ground line is 10-3/4°.

The wing is of semi-monocoque construction, with strength well distributed, attested by absence of wing failure after terrific battle damage. The two main spars have truss type construction rather than web type. The truss type spars, while more difficult to manufacture, are lighter and contribute about 30% of wing bending-strength.

The front spar is located at 15% of chord and joins the fuselage at an angle 6° off (rearward) of 90° from the plane's center line. The rear spar joins at the conventional 90°. Although the 6° angle of front spar incurs a penalty in requiring a heavier rib or compression strut at the wing root, it is more than offset by the space gained between the spars and straightline spanwise contour.

The truss type spars consist of 24ST tubing spar chords connected by web members of square, barrel, or rectangular tubing joined by means of gussets. Inboard sections of the chord tubes have an inside taper in wall thickness from .54 at the inboard ends to .13 at outboard ends. Outboard sections are of constant gauge square tubing.

The two spars are connected by truss type interspar ribs, spaced 15 to 18 in apart. Ribs are built up of 24ST channel chords and tubular diagonal members attached by gusset plates. Ribs, in turn, are joined to spars by riveted gussets. At places of heavy stress such as wing root terminal attachment points and landing gear mounting points, unusually heavy ribs or compression struts are provided. Heavier compression struts are also located at intervals among the lighter ribs to better distribute torsion.

Over the truss structure of the wing, and attached to it with skin type A17ST and 24ST rivets, 3/16 and larger, is a layer of corrugated 24ST, ranging in thickness from .064 inboard to .016 outboard, laid with the corrugations running parallel to the rear spar and riveted to the 24ST Alclad skin with flush type rivets forward of the front spar and with brazier head rivets aft of that line.

The wing is constructed in six sections: right and left inboard, right and left outboard, and tips. The entire assembly is attached to the fuselage by terminals of highly heat-treated, machined-steel forgings connected to the wing by bolts; heavy taper pins make up the terminal connections.

Inboard wing sections carry the engine nacelles, which are connected to the wing by bulkheads at the front spar and fairing angles around the wing-nacelle intersection. The inboard nacelles, housing the main wheels, have wheel wells reinforced by two large fore-and-aft heavy formed channels that tie into heavy steel landing gear support forgings which are securely attached to the wind surface and compression ribs. (This terminal connection of steel bolts and rivets aids in the nacelle attachment to the wing for the inboard nacelles only.)

Aluminum alloy terminals connected by taper pins are used at terminal connections between inboard and outboard wing panels. The wing tip has four terminal points of welded steel or magnesium alloy and is secured to the outboard wing panel by means of steel bolts.

Ailerons are the conventional fabric-covered D-nose type having an area of 60.2 sq ft to the hinge line, and an angular movement of 12° both up and down. Distance from plane of symmetry to centroid of aileron area is 38 ft, 10 in. The hinge centerline is located at 21.4% of the chord at the outboard hinge and at 27.3% of the chord at the inboard hinge. Mass balancing of the aileron is accomplished by a single weight on an arm extending forward from the aileron spar and concealed entirely within the wing contour.

The left wing only is provided with a trim tab. The tab has an area of 2.64 sq ft, angular movement both up and down of 15°, and is located at the inboard end of the aileron. The tab control knob is on the left side of the cockpit and requires approximately 3.76 turns for full operation.

Flaps are of the split type with span of 24 ft 4-15/16 in each, a chord of 34-13/32 in, and a total area of 139.1 sq ft. Maximum is 45° which increases the maximum lift coefficient of the entire plane from 1.53 to 2.0 and increases the drag coefficient from .1095 to .1775 at 1.2 stalling speed. It also increases gliding angle from 5° 39' to 7° 20' at 1.2 stalling speed, and decreases stalling speed 9 mph. Main spar of flap is a 24ST round tube, 2½ in in diameter, selected for efficient carrying of torsion and bending loads. Flap ribs are 24ST Alclad stampings which have stiffeners (intercostals) clipped to them, thereby eliminating notches in ribs and so retaining strength.

Flaps are actuated by means of interchangeable, irreversible screw and nut units operated through torque tubes, located adjacent to the rear wing spar, and powered by an electric motor. They can be raised or lowered at 126 mph in 15 to 30 sec.

Greatest single factor contributing to the Fortress' famed stability is the unusually large empennage. Although it was known at the time the original Fort was designed that a large vertical fin was essential to stability, and the original fin design was somewhat larger than the then accepted practice, it was puny by comparison with the present installation.

Some of the design features of the present dorsal fin were dictated by incorporation, in 1940, of a tail gun position, while other features were the result of redesign to achieve still greater stability. Pilots report stability so complete that the airplane can be flown on tabs alone without using the rudder, or with ailerons and elevators without using the rudder, or without resetting of trim tabs.

Horizontal tail surfaces have an area of 331.1 sq ft, span of 43 ft, maximum chord of 125 in, and a distance of 451 in from normal CG to 1/3 maximum chord point. The span places the tip of the stabilizer almost directly aft of the outboard engine nacelle.

The stabilizer has an area of 250.6 sq ft and is set at 0° relative to longitudinal axis. It is of two-spar construction with web-type spars made of 24ST web sheet and extruded spar chords, rolled 24ST spanwise stiffeners reinforcing interspar skin, and hydropressed 24ST ribs. The front spar extends 230 in to within 8 ft of tip end. The longer rear spar extends to the tip joining point.

From the point where the front spar stops to the end of the tip, the leading edge and rear spar take over as the shear-carrying structure. The Alclad horizontal stabilizer has skin lap joints. Attachment is by flush type rivets forward of the front spar and modified brazier head skin type rivets aft of that line. Beams are I type, and ribs are hydropressed. Both stabilizer panels are removable and attachment fittings are steel.

The elevator is constructed of a metallic frame covered with doped fabric, and is controlled by a short torque tube. Area is 115 sq ft including tabs and balance. Angular movement is 23° up and 14° down. Thirty% of elevator area is ahead of the hinge line. Elevators have uniformly distributed mass balance such that the chordwise elements are 100% statically balanced about the hinge centerline. Dynamic balance coefficient about the body centerline is approximately zero. Chord aerodynamic balance is 27%.

An aluminum alloy trim tab of 5.6 sq ft and with an angular movement of eleven degrees up and down is attached by piano hinge to each elevator, near the fuselage. External contour of the tab is flat on top and curved ¾ in maximum at center of 11-in chord on underside. The tabs have an irreversible control in lieu of mass balance,and the control wheel on the pilot's control stand, accessible to either pilot, requires 5.9 turns for complete tab operation.

Vertical tail surfaces have an area of 100.5 sq ft plus 42.4 sq ft in the dorsal or forward part, and the rudder has an area of 38.8 sq ft including tab and balance. Rudder angular movement, with stops compressed, is 22.° right and same left, with 21° available using cockpit control.

The dorsal portion is an integral part of the fuselage top and consists of hydropressed ribs and extruded stiffeners covered with 24ST Alclad. In manufacture it is assembled separately and joined to the fuselage as a completed assembly.

The vertical fin is similar in construction to the horizontal stabilizer: it is of two spar, web type, and formed of 24ST sheet and has 24ST extrusions for spar chords. Spanwise stiffeners are rolled 24ST and ribs are hydropressed stampings of 24ST sheet stock. A portion of the fin load is carried forward through the dorsal section.

The rudder has a metal framework with hydropressed nose ribs forward, and tail ribs aft, of the spar. The entire frame is covered with highly processed fabric which, when properly applied, provides a control surface having a minimum tendency to flutter because of dynamic air forces. Actuation is about three hinge points by a rudder torque tube controlled through a cable system from the cockpit.

The rudder has a uniformly distributed mass balance such that the chordwise elements are 100% statically balanced about the hinge centerline, and the dynamic balance coefficient about the body centerline is approximately zero. Chord aerodynamic balance is 25%.

The trim tab is aluminum alloy with an area of .34 sq ft and is installed at the the lower trailing edge. It has irreversible controls with the control wheel aft of the control stand in the pilot's cockpit. Approximately 6.9 turns of the wheel are required for complete tab operation. Angular movement is 22° each way.

Basically, the Fortress' fuselage has remained unchanged, although like the vertical fin assembly, it has undergone considerable modification and modernization. To provide for the added tail stinger, it was necessary to increase the diameter of the rear half of the fuselage. But while this obviously required dimensional increases in the structures, it did not require revision of the basic, original design formula.

The circular fuselage cross-section was adopted because it is more efficient from a standpoint of strength-weight ratio and ease of manufacture. In the latter respect, ability to form skin and stiffeners in continuous forming dies was a manufacturing factor which influenced design.

The fuselage is an all-metal, semi-monocoque structure with a maximum cross-section height of 103 in and maximum width of 90 in. Its structural design, like that of the wing, is such that strength is well distributed, and while built around rail-section longerons in the forward half, damage to one or more of them does not necessarily cause failure to the entire structure.

The main section, which includes the bomb bay, has three rail-section longerons of extruded 24ST, two in the top and one at the bottom to carry the load and reinforce the pilot's compartment which is a cutout in the top of the basic monocoque structure. All three rail-sections taper from the wing joining area, toward the front, reducing from I to T sections.

The lower longeron splices into the lower chord of the bomb bay beam on top of which is a catwalk. This truss, consisting of upper and lower chords and diagonals, built up of square 24ST tubing with extruded Ts and web members furnishes support for the center bomb as well as providing body bending continuity through the bomb bay.

Center bomb racks also serve as structural members giving column support to the lower chord of the catwalk truss or bomb bay beam. In the bomb bay, on both sides, are truss type body compression struts to carry part of the load past the interrupted longeron-stiffener structure, and to aid in transmitting wing torsion to the fuselage. To these compression struts are fastened the side bomb racks. At bulkheads 4 and 5, the ends of the bomb bay, crosswise strength is carried by four heat treated square steel tubes, two in each end forming continuations of the upper and lower wing spar chord loads. The steel tubes are connected trusswise by square tubing members of 24ST.

Fuselage load is carried in the portion just aft of the bomb bay by four rail-section longerons which taper out aft of station 6. The lower two provide additional reinforcing for the ball turret cutout and give the fuselage the required strength. The upper two longerons provide reinforcing for the radio compartment gun hatch.

Aft portion of the fuselage (about half), does not have the heavy rail-section longerons, depending for its strength upon a greater concentration of lighter extruded bulb angles and heavier circumferentials. All of the latter, Z-type, are formed from 24ST sheet and vary in spacing from 10 in forward of station 6 to 20 in aft of station 6, being heavier where spacing is wider.

For distributing concentrated loads from wing, tail gear, and empennage to the fuselage, bulkheads of varying design are used. They vary from solid — except for the passage door — to bulkheads with slightly more than unusually wide circumferentials. All are built up from flat sheet, rolled and stamped sections and extrusions, riveted together. In addition to imparting strength, they provide convenient supports for control linkage and equipment.

The truss around the bomb bay serves as the anchor point of the wing terminals, which are connected to the crosswise truss in the fore and aft ends of the bay, projecting outside the fuselage. A similar carry-through arrangement at bulkheads 8,9,and 10 in the aft section, supports the empennage assembly. The carry-through members are 24ST tubes and hydropressed sheet webs. The tail wheel structure is supported by bulkhead 7, which has two vertical members.

Front fin spar terminals are attached to bulkhead 9, and rear fin spar terminals attach to bulkhead 10. Stabilizer terminals are just outside the fuselage at stations 8 and 9 to which they attach.

Tail gunner's compartment, built as a separate component, relies primarily on skin and circumferential stiffeners for its strength, having no longitudinal stiffeners.

Framework of the plane consists of bulkheads and circumferential stiffeners, tied together throughout the length of the fuselage by longerons and longitudinal stiffeners aft to station 11; front and rear spars in the wing; crossties of nose, center, and tail ribs; and ribs and formers in the tail surfaces. A skin of 24ST Alclad is laid over this framework and fastened with aluminum alloy rivets. The thickness of the skin varies with the locality and depends on the amount of load carried.

On the wing, for instance, the skin — carrying two-thirds of the wing loads — is reinforced by corrugated dural which prevents buckling of skin between ribs. Additional reinforcement is also given to skin surrounding wheel well, exit, and access doors, and other cutouts in the structure.

Supports for the pilot's cockpit are large built-up beams under the floor, sloping aft to connect with the pilot's cabin door structure supporting the top turret immediately behind the pilot and co-pilot's seats and anchoring to bulkhead 4 below the cockpit.

Four means of emergency entrance and exit — in addition to pilot's and co-pilot's windows and the radio compartment hatch — are: releasable hinged floor in forward portion of fuselage; bomb bay doors; releasable hinged door near tail gunner's station, top right hand side; and releasable hinged door forward of bulkhead 7 on the right hand side. All except the bomb bay doors are operative from either inside or outside.

Pilot's compartment, reached through a doorway in the bulkhead between it and the bomb bay, is equipped to seat the commander pilot on the left and co-pilot on the right. Seats are adjustable vertically and fore and aft, and accommodate seat or back-type parachutes. Front window panels are fixed and leakproof: sliding type side panels are of shatter proof dehydrated glass, 1½ in thick, and fixed side panels are 5/16-in transparent plastic.

Engine nacelles are typical monocoque structures with four 24ST extruded longerons and several formed longitudinals located between the evenly spaced longerons. Longitudinal members are tied together with rolled sheet Z-section circumferentials, spaced about 10 in apart. Skin is 24ST except around exhaust stacks, where stainless steel is used. The firewall is also stainless steel.

Engine mounts are standard ring type of normalized X4130 steel tubing, arc-welded with X4130 steel forgings at the firewall connections. Each mount is interchangeable.

Spacing between fuselage fairing and wings is approximately 10 in to provide for the wing joint and is covered by hydropressed 24ST Alclad sheet fairing, held in place by means of machine screws and nut plates. The horizontal stabilizer, spaced with a 15-in gap between it and the fuselage, is also covered with fairing.

The Fortress was one of the first planes to "put the breeze to work" to take loads off of controls through the use of tabs, and one of the first to actuate retractable gear and other parts by means of electric motors as well as by manual drive shafts. This electrical installation has proven to be an outstanding member of that group of original innovations for which the plane has become famous. Combat damage, it was reasoned, would be less likely to put entire units out of commission if they were electrically controlled by means of dispersed and practically duplicate wiring systems. Thousands of instances of partial combat damage which failed to prevent operation have confirmed the soundness of that reasoning. Landing gear, flaps, and bomb bay doors are all electrically operated.

Manual control systems, likewise, are rather unique in that those to all control surfaces connect to and operate over quadrants, which makes them free-moving and easy to handle. Routing of cables is direct, with a minimum of pulleys and sharp angles in cable courses.

Landing gear is standard three-point type. Main gear is oleo-pneumatic, of Boeing design, with tread width of 21 ft 1½ in. The oleo strut is 9-19/32 in in diameter. Wheel diameter is 56 in, requiring a 16-ply tire, 56:19-23.

Main gear moves up and forward when retracting, leaving a small portion of the tire exposed when the gear is fully retracted. Tail wheel retracts fully. Brakes are dual hydraulic, obtaining pressure from an accumulator supplied in turn by an electrically driven pump and emergency hand pump.

Retracting mechanism for each leg consists of a retracting screw actuated by an electric motor attached to the screw by gearing and a manual drive shaft Retracting screws and nuts are interchangeable as units. Manual retracting shafts terminate on the rear side of the front wall of the bomb bay and are operative by the engine starter crank. Right and left main gear legs retract simultaneously when motor-driven, but individually when cranked.

Brakes are operative through pilot's or co-pilot's pedals, but the parking brake control is operative only by co-pilot and consists of a manually operated eccentric device which will hold the valves open. Tail wheel mechanism is similar to that of main gear and is similarly controlled.

Main wheel motion resulting from shock absorber deflection is along a line approximately 20° aft in the level landing position. Friction in the gearing and screw mechanism is sufficient for locking and for absorbing kinetic energy of the moving mechanism after power supply is cut off.

Engines are 9-cylinder Wright Cyclone, model R-1820, with 16:9 ratio from crankshaft to propeller shaft, and are rated at 1,200 hp each. They are equipped with Bendix Stromberg PD-12H2 injection carburetors and General Electric type B-22 turbosuperchargers.

The supercharger is installed in the engine exhaust system at the bottom of the nacelle. On the outboard nacelles the location is forward of the front spar, but on the inboard nacelles the location of the wheel well necessitates installation of the turbo (and intercooler) aft of the front spar.

The induction system is so designed that carburetor air must pass through the supercharger impeller and intercooler at all times. Exhaust gas pressure drives the impeller by flowing through a nozzle box where the gas is directed against a turbine wheel mounted on the lower end of the impeller shaft. Flow of exhaust gas through the turbine wheel is controlled by the waste gate in the nozzle box; thus, all or a part of the exhaust gas may be utilized to obtain the desired manifold pressure. The exhaust tail pipe arrangement ends at the turbo.

Carburetor air flows into the duct system at the wing leading edge, and passes to the supercharger impeller, by which it is compressed and forced through the intercooler into the carburetor. A relief valve is provided in the supercharger intake duct to permit the entrance of air to the supercharger if the flow through the inlet is accidentally restricted.

The inboard intercooler is located in the wing, directly aft of the nacelle, and the outboard intercooler is situated vertically in the nacelle, immediately behind the firewall. The purpose of he intercooler is to reduce the carburetor air heat resulting from compression by the supercharger. This reduction in temperature is accomplished by the passage of cold air around the intercooler from a second intake duct. The cooling air is then spilled overboard thru slots in the wing surface. The intercooler is similar to a coolant radiator or oil cooler.

One of the significant improvements on the Fortress is the addition of the Minneapolis-Honeywell electronic turbosupercharger control on the B-17G. This device, replacing the former engine oil regulator with its cable and pushrod controls, has three distinct advantages:

The complete installation consists of a single control knob and selector panel, pressure controls, turbo overspeed governors, amplifier units, and waste gate control motors. When the control is in operation the pilot sets the selector to a desired position and this in turn sets the pressure control. The governor controls the rate at which the change in turbo speed takes place and prevents overspeeding of the wheel. The amplifier provides current to the motor for closing or opening the waste gate as required.

Propellers are Hamilton Standard Hydromatic, controllable, full feathering 11 ft 7 in-diameter. Minimum clearances are: to ground, level landing, inboard, 17-9/32 in , outboard, 30-1/8 in; to fuselage, inboard, 9-3/8 in; to engine cowl, full feathered, ½ in; to leading edge of wing, 70 in; and inboard to outboard 25-7/17 in. Each propeller has its own feathering motor pump unit and draws oil for the pump from a standby reserve in the engine oil tank.

The engine cowling is made in three portions; the anti-drag or ring cowl, the cowl flaps, and the orange peel or inner cowl which is also called the accessory section cowl.

The engine ring cowling is hydropressed 24ST sheet, reinforced with spotwelded doubles, ribs, and extrusions along the edge of each of the three segments. The accessory section cowl is in five pieces and is made of stainless steel.

The fuel storage system includes 24 leakproof tanks. The largest hold 425 gal and one is installed in each inner wing panel between the nacelles and to the rear of the front spar. A 212 gal tank is installed in each inner wing between the 425 gal tank and the rear spar. A 213 gal tank is located one each side between spars, inboard of the inboard nacelle. Nine smaller feeder tanks (known as Tokyo Tanks) are located in each wing outboard of the outboard nacelles, four in the inner wing and five in the outer wing panel.

A droppable leakproof tank of 410 gal can be carried on each side of the bomb bay in place of corresponding bombs.

Fuel distribution is through four systems, each supplying one engine. By means of a reversible electrically driven fuel transfer pump, selector valves and interconnecting lines, fuel can be transferred from any auxiliary or engine tank to any other engine tank. Fuel booster pumps are installed in the outlets of four engine tanks to combat vapor lock at high altitudes and to supply extra fuel for takeoff.

An electrically controlled fuel shutoff valve is installed in the line between each fuel boost pump and the engine to prevent flow to a severed line in a nacelle or engine section. From the shutoff valve, fuel passes through a strainer mounted on the forward side of the firewall and enters the engine-driven fuel pump. From there it passes through the carburetor.

The oil system lubricates the engines, aids as a coolant in transferring heat from the engines, supplies hydraulic pressure to assist in propeller speed control, and feeds oil to the propellers, propeller governors and feathering pumps.

Each engine has its independent oil system. A self-sealing oil tank with a capacity of 37 gal is located in each nacelle aft of the firewall and is designed for a maximum diving angle of 25°

Oil cooler and temperature regulators are installed in the leading edge of the wings. The oil pump, Cuno filter, and scavenger pump are incorporated in the in the engine. Individual oil tanks of 1½-gal capacity and with ½-gal expansion space are provided for lubrication of each supercharger. Propeller feathering motors and pumps are mounted on the forward side of the firewall of each of the nacelles.

Oil flows from the tank by gravity and suction to the engine-driven oil pump which forces the oil under pressure through the filter to the engine. The oil then drops down to the sump, where it passes through a screen and is picked up by the engine-driven scavenger pump, forced through the oil cooler and returned to the tank.

Propeller feathering oil is obtained from the oil tank sump from a stand-by reserve. The feathering pump draws the oil from this source and forces it under pressure to the feathering valve in the propeller governor.

An oil dilution fuel line enters the main oil out line from the tank at a Y cock drain valve.

The electrical system is 24 VDC, the primary sources for which are four 200-amp engine-driven generators and three 24-V batteries. Negative terminals of the generators and batteries are grounded to the airplane structure, and all circuits are single-wire with ground return.

Secondary power sources are two 400-cycle, 26-225 VAC inverters, a 3 VAC transformer, and an auxiliary 2,000-watt gasoline engine-driven DC generator.

The electrical system is divided into 14 different circuits, each identified by a code letter for easy and accurate maintenance and repair.

The DC power circuit includes, in addition to the four generators and three batteries, 4 voltage regulators, 4 reverse-current relays, 3 battery solenoids, and the power distribution buses. The 4 generator and 3 battery switches, 4 ammeters, voltmeter, and voltmeter selector switch are located on pilot's control panel on the left sidewall. The master ignition switch which turns off all battery power and grounds the ignition system to stop engines, is located on the central control stand between pilot and co-pilot.

The three 34-amp-hr. batteries are connected in parallel with the four engine-driven generators. The positive terminals of the batteries are connected to inboard nacelle junction shield buses by type B-4 solenoid switches, which in turn are controlled by three type B-5A battery switches on pilot's control panel. The battery switch circuits are protected by 15-amp fuses in inboard nacelle junction shields. The negative sides of the battery solenoids are grounded through the master ignition switch when the switch is on.

Output voltage of each generator is maintained at 28.5 V by the voltage regulator. The load is equally divided among the generators by an equalizer coil in each voltage regulator. Each reverse current relay is adjusted to connect the generator to the power system when sufficient voltage has been built up, and to prevent battery discharge when the generator voltage falls below battery voltage by opening the circuit when approximately 10 amp reverse current flows.

One ammeter for each of the four engine-driven generators is connected to a 50 mV, 300 amp shunt in the generator relay shield in each nacelle. The shunt is inserted in the ground lead. A voltmeter controlled by a 4-position selector switch on pilot's control panel measures the voltage of any of the generators.

External power from a battery cart or portable generator may be supplied through a 3-prong receptacle in the lower side of the fuselage aft of the forward entrance door. A hinged door covers the box, which contains a 3-hole to 2-hole adapter plug for use with British equipment.

A fuse panel on the rear bulkhead of pilot's compartment contains 45 of the 77 fuses in the plane. In addition, the landing gear indicating relay is mounted on a bracket on the panel. A small auxiliary panel contains 5 fuses and control relay for the carburetor air filter motors, indicator lights, and switch; and 4 fuses for bomber's and pilot's windshield. Still another panel, beside the lower turret, contains 11 fuses for electrical and radio equipment in the rear portion of the plane.

Landing gear motors are all controlled by switches at pilot's station. The motors are mounted at the upper end of the retracting mechanism in the inboard nacelles, and receive unfused power through two solenoids and actuate the gear through a planetary 40:1 reduction gear, clutch assembly,and solenoid engaging mechanism.

Tail wheel retracting motor is mounted above the wheel and actuates it through a clutch and reduction gear, and solenoid engaging jaw. Means for manual control are also provided.

Bomb bay doors are actuated by a single motor mounted at the forward end of the bay, on the left side of the catwalk. Solenoids are mounted on an adjacent bulkhead. The doors may be opened with a hand crank inserted through a hole in the step at the forward end of the catwalk.

Other electrical circuits are: bomb control, deicer and pump, flight control instrument, interior and exterior lighting, starter, propeller feathering and warning signals. All are installed in open group rather than conduit to expedite repair of gunfire damage, in flight as well as on the ground. To increase reliability, duplicate circuits are provided for bomb controls.

The B-17 carries equipment for long and short range two-way voice and code communication; emergency transmission; reception of weather, range, and marker beacon signals' directional indication; and interphone communication between crew members.

In addition, some Flying Fortresses are equipped with instrument approach equipment.

Equipment normally installed is the following, or equivalent: Interphone — RC-36; command radio; liaison radio — SCR-287-A; radio compass — SCR-269-G; marker beacon — RC-43; and emergency radio — SCR-578.

Propeller anti-icing equipment consists of two electric-motor-driven fluid-metering pumps located beneath the radio compartment floor at the forward end of the camera pit. Fluid is obtained from a 20-gal tank, and each pump supplies two propellers.

Vacuum and deicer system equipment includes two vacuum pumps mounted on the accessory case of No 2 and No 3 engines.

The B-17's hydraulic system operates only the cowl flaps and brakes. It has an emergency system for operation of the brakes in event of pressure failure in the main system. Operating pressure is 600-800 psi, developed by an electrically driven pump.

The heating system is operated on hot air which is transferred from a glycol system installed in No 2 nacelle. Heating system fluid (55% diethylene glycol, 45% ethylene glycol, by weight) is stored in a tank located in the top of nacelle No 2, and flows to the engine-driven pump which circulates it at a rate of 55 to 60 US gal per hr. The flow is directed to a filter which removes impurities and the fluid is then pumped through three boilers which are installed in series in the exhaust stack.

A relief valve, between pump and filter, bypasses the glycol back to the supply line if high pressure is built up in the system. Circulation of the glycol is continuous and therefore it must be constantly cooled. For this purpose, a radiator is installed between spars in the left hand wing gap. Ram air from the intercooler air inlet absorbs heat from the glycol at the radiator and passes through the radiator and into the cabin A controllable damper in the radiator may be operated to spill the air overboard, if desired.

Four independent low-pressure oxygen systems operating at maximum pressure of 425 psi are provided, some B-17s having as many as 16 outlets. Each system supplies a portion of the crew and is separate from the other systems, thereby reducing the possibility of complete failure under combat conditions.

The system is supplied by 18 type G-1 bottles, each containing approximately 4-hr supply for one man at 30,000 ft. Thirteen brackets are provided in convenient locations for 10 portable walk-around bottles.

Armor plate protects pilot, co-pilot, top gunner, tail gunner, and side gunners.

Concurrently, the Fortress is armed with 13 .50-cal machine guns, eight of them arranged in pairs in four turrets — chin, top, ball, and tail. Five single .50s are fitted, one in the top of the radio compartment, two amidships or waist positions, and two in the nose section.

Bomb rackage varies but now includes both internal and external bombs of 100, 300 500, 1,000, and 2,000 lb. The bomb load capacity likewise varies greatly, depending upon the range desired. Range with maximum bomb load is limited.

The Boeing Flying Fortress has proven its right to a place in US military history alongside those sluggers of this and other wars. And it has piled up a vast store of data upon which the design of the Superfortress has been based.

This article was originally published in the January, 1945, issue of Aviation magazine, vol 44, no 1, pp 121-144.
The PDF of this article [ PDF, 60.2 MiB ] includes 7 photos, a 3-view and 29 detail drawings and diagrams, and 16 data tables. Two of the photos and 4 of the drawings are duplicated in a full-sized image of the foldout (pp 129-132 in the original), plus an image of a drawing of the skeleton of the plane from the foldout. Photos are not credited.