The B-25 Mitchell

New chapters that are being written daily over each of the blazing battlefronts of the world in the saga of B-25 Mitchell bombers are focusing increased attention of engineers and the public alike on these multi-purpose planes manufactured by North American Aviation, Inc.

The Mitchells haven't missed a "show" in this war, whether it has been in the Atlantic, Pacific or Mediterranean or over island or continent. Built for more than two years in quantity at North American's Inglewood and Kansas City divisions, the B-25s are "standard equipment" in the air forces of not only our nation but Britain, Russia, the Netherlands East Indies and China as well.

Built for the pulverizing type of "level bombing" at medium altitudes, the big, armored bombers have proved their versatility at troop-strafing, night fighting, as torpedo planes, transports, long distance fighters, and reconnaissance planes. Their short takeoff run, sturdy structure and rugged tricycle landing gear contribute to their ability to land on rough, improvised, frontline fields, an asset that is essential in wartime flying.

Armed with a large number of 0.50-calibre machine guns, carrying a crew of five men, the Mitchells can fly long distances, plaster targets with tons of bombs, torpedoes or depth bombs; can knock pursuing Messerschmitts or Zeros out of the air with ease and get home safely, even after absorbing deadly punishment. For short range work, with fuel tonnage replaced by bristling armament, they have carried a large number of guns, with 400 rounds of cartridges per gun, protected by nearly 800 pounds of armor plate.

These fast, powerful bombers spearheaded the Allies' aerial assault which preceded the invasion of Sicily, after striking with deadly effect in driving Marshal Erwin Rommel out of Africa. From India and Burma communiques bring daily word of their destructive raids deep in Jap-held territory. From Russia, in a terse military communique, comes the characterization "highly effective." In the Aleutians, they have consistently bombed Kiska and helped in shattering the Japanese foothold in Attu with high explosive, fragmentation, and incendiary bombs.

Besides knocking down Zeros and bombing Jap airdromes in the Chinese theater, they have made numerous long distance surprise raids on Hong Kong, on Canton, and other enemy centers. They are being mentioned daily in General MacArthur's dispatches, many of which tell of the B-25s being flown by Dutch flyers.

In the Bismarck Sea battle, when Allied forces wiped out a fleet of 22 Japanese ships, the B-25s stormed in mast-high, planted delayed fuse bombs, blasted warships and transports, again accomplishing tasks not originally conceived when the airplanes were first designed.

Most sensational of America's early-day war exploits, and still rated among the greatest events of the war, was the raid on Tokyo led by Major General Jimmy Doolittle. The Army Air Force chose 16 B-25s for the unprecedented feat of a squadron of 14-ton bombers taking off, fully loaded, from an aircraft carrier, and making the long overseas hop that was climaxed by the sudden, swift attack on the great industrial centers of Japan.

In December of 1941, a B-25 became the first US Army airplane to sink an Axis submarine. In April of 1942, ten of the B-25s participated in the longest, most daring mass bombing raid in history up to that time. Taking off from Australia, they flew 4000 miles to and from the embattled Philippines, in company with three B-17s. Using secret bases, they sank four Jap transports, shot down many enemy airplanes, bombed bases, airports and troop concentrations.

Certainly a tribute to the B-25s ease in handling, the Army Air Forces chose one of them on June 23, 1943, as the first bomber ever to be ferried across country by a woman pilot and copilot. Then, too, the Office of War Information has termed the Mitchell the "world's best medium bomber."

On July 10, 1943, the Truman Committee, in its appraisal of American aircraft, found the North American B-25 Mitchell "a valuable plane and the rate of production very substantial."

The first B-25 was built when North American successfully submitted one of the two designs chosen by the Army Air Forces after circularizing the American aircraft industry in an effort to create a medium bomber force as part of a potential aerial striking weapon. It required 8500 original drawings involving expenditure of 195,000 engineering man hours. Since that time more than 11,000 design drawings have been devoted to improving the bomber, requiring in excess of half a million engineering man hours, to keep up with revolutionary advances in air-war science.

Ease and rapidity of construction and repair were one of the dominating thoughts in the airplane's design. The structure was divided into forty-eight major, separable components, contracts for which could be divided among more than 900 suppliers and subcontractors, if need be, all over the country for expediting quantity manufacture.

Certainly in the months America has been pitting its airplanes against those of the Axis "something has been added" to this big, tough bomber, which has been employed in various ways, from carrying gargantuan overloads long distances to Hong Kong, Canton or Tokyo, to battling it out with swarms of Messerschmitt fighters, and all with the flight responses and the easy controls of a light training plane.

North American was awarded a contract for the B-25 September 20, 1939, and a completed bomber structure was ready on July 4, 1940, for static test at Wright Field. The first B-25 was test flown on August 19, 1940.

In the models that have followed, changes in range and armament have been made in response to the constantly shifting tactical requirements of such an airplane. In the latter part of 1940, for example, changes were made in the type of fuel tanks used, armor plate was added, this model becoming known as the B-25A. Next engineers redesigned the new fuselage of the bomber to incorporate power-driven gun turrets, designating the model the B-25B.

The Mitchell in combat today is a twin-tail, high-speed, mid-wing land monoplane, powered by two 1700-horsepower Wright Cyclone, supercharged 14-cylinder, twin-row, air-cooled radial engines, driving 12' 7" full-feathering, constant-speed Hamilton Standard Hydromatic three-blade propellers. It can carry about 5000 pounds of bombs. including torpedoes or depth charges, six 0.50-calibre machine guns with 2400 rounds of ammunition, and a crew of: pilot, copilot, bombardier, radio operator and gunner, at a speed in excess of 300 miles an hour for long distances. For fly-away delivery to war zones, using droppable tanks, its range exceeds 3000 miles. Its weight empty is 20,000 pounds, normal gross weight 27,000 pounds, useful load nearly 7000 pounds.

Wings

The full-cantilever, stressed-skin wings, consist of a center section (integral with the fuselage), and two removable single-spar outer panels, with detachable tips. The center section wings carry the power plants with their nacelles, the self-sealing fuel and oil tanks and such additional equipment as oxygen bottles and fire extinguishers. The center section is a strong, rigid structure, built on two heavy, reinforced, full-length spars.

The wing outer panel, including the aileron and large oil cooler assembly at the inboard end, weighs 560 pounds, has a maximum chord of about eight feet, and is approximately twenty feet long, excluding the detachable tip. This structure is of 24ST aluminum alloy extrusions and Alclad sheet, consisting of spars, power-pressed ribs and skin stiffened in a span-wise direction by stringers.

Total wing span is 67' 6.7"; area is approximately 610 sq ft.

Sealed-type ailerons, each 11' 7" long, have combination booster and controllable trim tabs, and are operated by control wheels in the pilot's enclosure with cables that lead out of the torque tube on each side of the airplane to the aileron sector on each rear wing spar. A cable movement of 3½ inches will move one aileron up 30° and the other down 15°. Controls in one wing can be shot away without affecting the controls in the other wing.

Slotted type, trailing edge flaps on each wing (15 feet long) in two sections on each side of the engine nacelles, are operated by hydraulic struts in the nacelles connected to torque tubes, or in an emergency, by hand cranking.

Fuselage

The fuselage is a semi-monocoque, all-metal, four-longeron stressed-skin structure, 53 feet long, consisting of three main sections: front, center and rear. The forward section contains the bombardier's "greenhouse," and the compartments for pilots and navigator. The intermediate section (built integral with the wing center section) consists of the bomb bay with hydraulically operated doors. The rear section contains the radio and gun turret stations, and the lie-down observation post in the transparent tail cone. Passageways under the pilot's compartment and over the bomb bay provide interchangeability of crew members. Two main entrance hatches are in the navigator's and the photographer's compartments. Emergency exit hatches are provided at the stations of bombardier, pilot and photographer.

Bombardier's Station

Starting at the nose of the ship there is the Plexiglas enclosed bombardier's compartment, equipped with bomb controls, bombsight and two 0.50-calibre machine guns, one fixed and the other flexible in a centered ball-and-socket joint.

Pilot's Cockpit

Aft of this, accessible for the bombardier through a lower crawl way, is the cockpit for pilot and copilot, with controls for flight, engine, landing gear, wing flaps and cowl flaps, emergency bomb release, remote control for the command radio and a button for firing the fixed nose gun. Here also are controls for the recognition lights for signaling to shore stations and ships. The automatic pilot is installed in the center of the pilot's instrument panel.

The flush-type cockpit enclosure has an emergency exit at the top, windshield and movable panels of laminated glass and fixed panels of non-inflammable transparent sheet. Ingress and egress is through the entrance hatch in the navigator's compartment.

Navigator's Station

A curtain covers the opening from the cockpit into the navigator's compartment behind the pilot. Here is a navigator's chart table with protractor, the drift sight, and astro-compass. The drift sight consists of a telescopic sight, with controls for measuring the drift of the plane in relation to the path of flight. Twelve drift signals of both day and night type are available. When a signal is dropped, the drift of the airplane is shown by comparison with the source of the smoke (by day) or of the light by night.

In it are the astrodome and astro-compass for obtaining the true bearing of distant objects. With it the navigator can compare the theoretical route with the actual. The astrograph projects the corresponding celestial maps on the chart table, which is equipped with special plotting paper and drafting machine for laying out an accurate course. The remote reading compass transmitter in the leading edge of the wing, away from magnetic interference, transmits its data to dials in front of the pilot and navigator.

Just aft of the bomb bay and interconnected to the navigator's compartment by the bomb bay crawl deck is the radio operator's compartment. This section houses all the radio equipment in use on the B-25 bombers. The compartment also contains the upper and lower power-operated turrets, scanning windows and emergency escape window.

From the turrets an armor plate door leads aft to the tail compartment and rear entrance hatch. There are floor and side-window camera settings (for charting, night, motion picture and vertical mapping cameras). In earlier models a gunner sat upright in the transparent tail cone.

A single hydraulic system, using an operating pressure of 1000 psi is employed for concurrent operation of the main landing gear and nose gear and for the selective operation of wing flaps, engine cowl flaps and bomb bay doors. The system incorporates a pressure storage of 1150 pounds for operation of the brakes. Two engine-driven pumps, one on each engine, furnish the pressure and are capable of individual operation of the system. For ground and emergency use, a hand pump can be used effectively, even though the supply for the engine pumps is lost. The turret guns are charged hydraulically from the general system. Two pressure accumulators are in the navigator's cabin, one for the general supply and the other for the brakes.

Landing Gear

The fully retractable, hydraulically-operated tricycle landing gear, equipped with air-oil shock struts, is enclosed by doors in the up position. The main gear, with its 47-inch wheels, retracts into the engine nacelles, and the nose gear with 30-inch wheel into the fuselage. In case of power failure the gear can be lowered manually. The tail skid is non-retractable.

A turn indicator tells if the nose wheel is turned 15° in either direction while taxiing. An hydraulic shimmy damper, mounted on the nose wheel strut, prevents shimmying when taxiing or landing. When the yellow warning flags on the pilot's panel disappear, it shows that the gear is locked in the down position.

Brakes, hydraulically operated by means of toe-type pedals, also have air brake power at 550 to 600 psi for emergency.

Electrical System

A direct current, single-wire electrical system for operation of the radio, lights, electric motors, starters and for certain instruments, is provided by a 24 V battery inside each engine nacelle, charged by 200 A, 30 V generators on each engine. Either battery will operate the electrical system, including the starters.

Battery-driven rotary converters, located in the bomb bay passageway, supply the alternating current required by some units. The converters are of 750 VA capacity, and change the direct current to 400 Hz alternating current, which is used for the fluorescent lights, radio compass Autosyn instruments, and compass illumination. There are remote-indicating engine instrument transmitters in each nacelle, connected to indicators for oil and manifold pressure, oil temperature and tachometer. Electrical indicators are provided for fuel level, landing gear and wing flap position.

Exterior lighting consists of two landing lights, a passing light and six position lights. Red, green, and amber recognition lights beneath the right wing tip, and a clear lamp above, are installed to flash signals or burn steadily. Cockpit, dome and extension lights are provided for the various stations. Fluorescent lights for the pilot's instrument panel are powered by 115 VAC from the converter. Conventional lights are also provided. Fluorescent markings on charts, controls and panels make them visible under ultraviolet light, when the cabins are blacked out in night operations.

The portion of the electrical system controlling power plant operations consists of engine starters, primers, the ignition system, propeller feathering pumps, and oil dilution solenoids.

The feathering consists of push button solenoid switches on the pedestal switch panel, and a feathering pump motor in each nacelle.

Eight outlets are provided for plugging in electrically-heated flying suits. Visual and audible electrical warning systems are connected with the landing gear, lower turret and trailing antenna systems. to warn of unsafe landing conditions.

The upper and lower Bendix power-driven gun turrets are operated from the airplane's electrical system. A blue lamp on the pilot's switch panel warns if the nose gear is down and when the lower turret is extended. Other electrically operated units are the pilot static tube heater, de-icer and anti-icer motors, tow target windlass, electrical control for combustion type cabin heater.

The radio equipment consists of the following: the command set for plane-to-plane communication; medium range liaison set for airplane-to-ground messages; frequency meter; the radio compass receiver, used for direction finding in cross country navigation; the marker beacon receiver, which operates in conjunction with the instrument landing system; and multi-place interphone lines to each crew station, together with dynamotors, mouth and throat microphones.

This article was originally published in the November, 1943, issue of Air Tech magazine, vol 3, no 5, pp 21-33, 66, 68.
The original article includes 4 photos and a three-view and 17 schematic drawings of airplane systems.
Photos and drawings are credited to North American Aviation.

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