North American B-25 Mitchell B-25 (NA-62)

The NA-40 had lost the light bomber competition, but it gave North American the basis for an entry in a different one. On 11 March 1939 the Air Corps issued Proposal No. 39-640 for a medium bomber able to carry 3,000 lb of bombs over 2,000 miles at more than 300 mph, powered by the Pratt & Whitney R-2800, the Wright R-2600 or the Wright R-3350. The request went out across the industry and Martin, Douglas, Stearman and North American replied. Lee Atwood, the company's vice president and chief engineer, took charge of the project, which received the company designation NA-62.

The NA-62 drew heavily on the NA-40B and kept its basic layout, but it was a somewhat larger aeroplane with more speed, range and payload: some ten square feet more wing area, a fuselage six feet longer and a gross weight of 28,000 lb against the 20,000 lb of the NA-40B. To take the heavier bomb load, the raised tandem cockpit was replaced by a side-by-side flight deck whose top line matched the top of the fuselage; that widened the fuselage and meant the bombardier reached the nose through a crawl tunnel beneath the pilots. The wing came down from the shoulder to a mid-fuselage position while keeping the continuous three-degree dihedral of the original design. A NACA 23017 aerofoil was chosen at the root, changing to a NACA 4409-R at the tip, with a slight reflex in the outer trailing edge to soften the stall. The Wright Cyclone nacelles were extended aft of the wing and the main wheels retracted backwards into them. Compared with the rival Martin Model 179, the North American design built in features for easy field maintenance and repair.

The crew numbered five: pilot, co-pilot, bombardier, navigator/radio operator and gunner. Defensive armament consisted of four machine guns. One 0.30 in gun in the nose could be fitted to any of three ball-and-socket mounts, one facing forward and one to each side; a second 0.30 in gun sat behind a Plexiglas hatch in the upper rear fuselage; a third could be fired from either waist window or through an opening in the floor; and a 0.50 in gun occupied the extreme tail inside a streamlined transparent canopy with clamshell doors that opened sideways to let the gun traverse, the tail gunner lying prone. Maximum offensive load was 3,600 lb (1,600 kg) of bombs.

The proposal met the Army's 10 September deadline. The service was sufficiently impressed to issue a contract on 10 August 1939 for 184 NA-62s under the designation B-25, finally approved on 10 September; the Martin 179 received a contract for 201 aircraft as the B-26. Because the aircraft had been ordered off the drawing board there was no XB-25 and no YB-25. A full-scale mock-up was approved on 9 November 1939, and the first B-25, serial 40-2165, was in final assembly by early summer 1940. It made its first flight on 19 August 1940 with test pilot Vance Breese at the controls and engineer Roy Ferren in the right-hand seat; on that flight Ferren noted a severe roll-yaw coupling.

Two of the early aircraft went to Wright Field for tests while North American kept the first. The trials confirmed the problem: the constant-dihedral wing gave directional instability, including Dutch roll, and produced banked rudder turns, which the Army found unacceptable because it required flat rudder turns for small heading corrections on the bomb run. The cure was a cranked or "gull" wing in which the panels outboard of the nacelles were set horizontal. It has been reported that the change came in on the tenth aircraft built, though no records confirm it; what is certain is that the first nine left the factory with constant dihedral and that a test flight on 25 February 1941, during acceptance trials, confirmed the improvement. The gull wing was retained through the entire Mitchell production run. In parallel the vertical tails went through several shapes: from the outline inherited from the NA-40B to two rather ungainly rectangular fins, then a flattened triangle, and finally the back-tilted fin and rudder that became standard.

The name "Mitchell", honouring General Billy Mitchell, appears to have been Lee Atwood's idea, and the Air Corps readily agreed.

Twenty-four B-25s were built, serials 40-2165 to 40-2188 and company numbers 62-2834 to 62-2857. The first was accepted by the Army in February 1941 and the first recipient was the 17th Bombardment Group at McChord Field, Washington, which ultimately took 19; one each went to Chanute and Lowry Fields and a couple stayed with the company. Power came from two Wright R-2600-9 Double Cyclone fourteen-cylinder radials rated at 1,700 hp for take-off and 1,500 hp at 2,400 rpm. Maximum speed was 322 mph at 15,000 ft, service ceiling 30,000 ft and range 2,000 miles with 3,000 lb of bombs. Empty weight was 17,258 lb and maximum weight 28,557 lb, on a wingspan of 67 ft 6.7 in, a length of 54 ft 1 in, a height of 16 ft 4 in and a wing area of 610 sq ft. Fuel was carried in two forward wing tanks totalling 484 gallons, two rear wing tanks totalling 432 gallons and a droppable 420-gallon bomb-bay tank.

Once its test programme was over, the first B-25 (40-2165) was converted by North American into a company transport: all military equipment came out, seven seats went into the fuselage, the bomb bay became a baggage hold, a bunk was fitted in the crawl space above it, windows were cut into the sides and the glazed nose was faired over completely. It was written off in a crash-landing on 8 January 1945 during a routine check flight, the crew unhurt. In 1943 the 40-2168, one of the nine constant-dihedral aircraft, was modified in the same way as General Arnold's personal aeroplane, with its wing panels revised to zero dihedral. How much the general actually used it is unknown; after the war it was sold as surplus, passed through several owners including Howard Hughes, and remains airworthy as N2825B.

The 24 initial B-25s were less an operational batch than a flying test bed on which the gull wing, the definitive tail and the five-man formula were settled, a formula North American would refine in the B-25A and, above all, in the power-turreted B-25B.