The North American B-25 Mitchell is a twin-engined medium bomber designed and produced by the American aircraft manufacturer North American Aviation. It was named in honor of Brigadier General William "Billy" Mitchell, a pioneer of US military aviation.
The B-25's design can be traced back to the late 1930s, having been designed as the NA-62 (based on the preceding NA-40B) to fulfil a requirement issued by the United States Army Air Corps (USAAC). While not the winning submission, nevertheless, the USAAC issued a production order on 20 September 1939 for the type. The prototype performed its maiden flight on 19 August 1940; shortly thereafter, a redesigned gull wing was adopted to correct a tendency to Dutch roll. The B-25 was a relatively forgiving aircraft to fly and durable under fire.
The B-25 entered service with the United States Army Air Forces (USAAF) in early 1941. Produced throughout World War II, numerous variants of the type were used operationally, contributing to nearly 10,000 B-25s being built. It was the most-produced American medium bomber and the third-most-produced American bomber overall. These included several limited models such as the F-10 reconnaissance aircraft, the AT-24 crew trainer, and the United States Marine Corps' PBJ-1 patrol bomber.
The B-25 was active in every theater of the conflict, including the Pacific War, the Second Sino-Japanese War, the North Africa Campaign, and the Italian Campaign. In addition to the USAAF, the B-25 was operated by many Allied air forces, including the Royal Air Force and the Soviet Air Force, often being supplied under Lend-Lease. Following World War II, many B-25s continued in service into the 1950s, albeit typically relegated to training, reconnaissance, and support roles; the United States Air Force performed its final flight of the type in 1960. Overseas, the B-25 would also be used by various operators in further wars, including the Nigerian Civil War and the Indonesian National Revolution.
On 11 March 1939, the United States Army Air Corps (USAAC) issued Proposal No. 39-640 specifying a medium bomber capable of carrying a 3,000 lb (1,400 kg) bombload over a range of 2,000 miles (3,200 km) at a maximum speed in excess of 300 mph (480 km/h). It was also stipulated that the envisioned aircraft ought to be powered by a pair of specific engines, these being the Wright R-2600-92 Twin Cyclone, the Wright R-2800, and the Wright R-3350. Furthermore, a relatively short development schedule was specified; in addition to the selection being made within 6 months of issuing, the winner would be expected to commence delivery of production aircraft within 24 months of the production contract's issuing.
North American Aviation (NAA) opted to base its response on the pre-existing NA-40B design, which was refined into the larger and heavier NA-62 proposal. Specific changes from the NA-40B included the adoption of a wider fuselage to accommodate a larger bomb bay, the use of a mid-mounted wing, a more conventional side-by-side cockpit arrangement, extension of the engine nacelle fairings, and the selection of a twin-fin tail unit. The aircraft used the NACA 23017 airfoil at the wing root changing to a NACA 4409-R at the wingtip. In contrast to the competing Martin Model 179 proposal, the North American design included easy field maintenance and repair features, and according to Avery, "It promised to be an easy airplane to fly and placed no special requirements on pilot training programs."
In August 1939, it was announced that, following a competitive evaluation of the six submissions received, it was determined that the rival Model 179 was superior to all other proposals and thus Martin was awarded an initial production contract for the aircraft, which was designated B-26. NAA's submission, which had been found to be the second-best proposal during the evaluation stage, was not in vain. Partially due to the urgency attached to the requirement, and partially attributable to Martin declaring that they were unlikely to be capable of providing all 385 aircraft within the allocated timeframe, NAA received an initial production contract, No. W353-ac-13258, for the delivery of 184 aircraft powered by the Wright R-2600; the type was designated B-25.
On 19 August 1940, Vance Breese and NAA test engineer Roy Ferren flew the first flight test, during which Ferren noted a severe roll-yaw condition. During preliminary flights performed by USAAC pilots, it was noted that the aircraft exhibited a tendency to Dutch roll, accentuated by wind and gusts, and demanded this be rectified. NAA's first nine aircraft had a constant-dihedral, with the wing having a consistent upward angle from the fuselage to the wingtip. Eliminating the outer wing panels' dihedral flattened the wing, giving it a distinctive gull wing configuration and solving the aerodynamic problem. A test flight conducted amid the aircraft's acceptance trials on 25 February 1941 confirmed that the redesigned wing had improved the flight characteristics. The vertical tail also went through five variations before being finalized, the fin area being more squared-off on the final design. The trials were considered to be largely successful.
The B-25 was considered to be a relatively safe and forgiving aircraft to fly. With one engine out, 60° banking turns into the dead engine were possible, and control could be easily maintained down to 145 mph (230 km/h). The pilot had to remember to maintain engine-out directional control at low speeds after takeoff with rudder; if this maneuver were attempted with ailerons, the aircraft could snap out of control. The tricycle landing gear arrangement made for excellent visibility while taxiing. The only significant complaint about the B-25 was its extremely noisy engines; as a result, many pilots eventually suffered from some degree of hearing loss. A Clayton S stack, introduced to quench the exhaust flame, was introduced in the B-25C series. These stacks protruded through the cowling, and though they weighed less than the replaced collector ring, they reduced aircraft speed by 9 mph (14 km/h) due to the required aircraft fairings. According to Avery, "The increase in noise as compared to collector rings ported on the outboard side of the nacelles was a general crew complaint."
The B-25 was exceptionally sturdy and could withstand tremendous punishment. One B-25C of the 321st Bomb Group was nicknamed "Patches" because its crew chief painted all the aircraft's flak hole patches with bright yellow zinc chromate primer. By the end of the war, this aircraft had completed over 300 missions, had been belly-landed six times, and had over 400 patched holes. The airframe of "Patches" was so distorted from battle damage that straight-and-level flight required 8° of left aileron trim and 6° of right rudder, causing the aircraft to "crab" sideways across the sky.
The first large production order, for 1,025 B-25Cs, was placed in September 1940 and subsequently increased by a further 1,000 aircraft. In June 1941, another large order, for 1,200 B-25Ds, was placed, it would be later increased by 1,090 aircraft. Development of further models was also underway at this point. Despite these efforts, by the time of the attack on Pearl Harbor, only 130 B-25s had been delivered.
Special variations were made to accommodate photo reconnaissance, armament, long range ferry, anti-submarine patrol, winterizing, and use in a desert environment. By February 1941, the first 24 B-25s were configured with three .50 caliber guns and a single .50 caliber tail gun. The B-25A had self-sealing fuel cells. The B-25B had top and bottom turrets with twin .50 caliber guns each, though the tail gun was removed. By December 1941, the B-25C had additional self-sealing fuel cells outboard the wing center section. By February 1942, the first B-25D, and then in May 1943, the 75 mm (3.0 in) cannon-armed B-25G series were accepted by the Air Corps. By August 1943, the B-25H had a lighter 75 mm cannon, four nose guns instead of two, two waist guns, two in the tail turret, four blister gun packs, and eliminated the co-pilot after Jimmy Doolittle questioned the need. In December 1943, the B-25J was introduced, the final variant and the most produced, reincorporated the co-pilot position and included a bombardier.