The Boeing/ Stearman XA-21 was a twin-engine attack aircraft that was one of the most advanced aircraft to emerge from the Stearman Company.
The Stearman Model X-100 was designed in response to a US Army attack aircraft specification. Most of the work was done while Stearman was an independent company, but the design was completed after Stearman was taken over by Boeing in 1939.
The Model X-100 was a three-man aircraft with room for a fourth. It was the first Stearman monoplane and the first Stearman aircraft to use all-metal construction. It was a high wing monoplane, powered by two Pratt & Whitney R-2180-S1A1-G engines. These could deliver 1,150hp at 2,350rpm in normal flight and 1,400hp at 2,500rpm at take-off. The X-100 was the only aircraft to use the R-2180 engine.
The X-100 had a tall but narrow straight sided fuselage. As originally built it had a rounded glazed nose (as used on the B-29), with the bombardier in the lower part of the nose and the pilot above and behind him. This was later changed to a standard 'stepped' nose, with a windscreen close to the pilot and a glass roof just above the bombardier. The gunner/ radio operator had a radio compartment in the rear fuselage and a gun turret just ahead.
The X-100 had a number of advanced features. It had integral fuel tanks, built as part of the structure. The outer wing panels and central fuselage were sealed to act as flotation devices in case the aircraft was forced to ditch. The undercarriage was electrically-actuated. It had fully-feathering constant-speed propellers.
The X-100 was delivered to the Army in September 1939, by which time Stearman was part of Boeing. It was given the Army designation XA-21. Despite being quite an advanced design, it was the least successful of four designs submitted to the Army at this time. It was up against the North American NA-40, which became the North American B-25, the Martin 167F, which became the Martin Maryland and the Douglas Model 7B, which became the DB-7/ A-20/ Havoc. The X-100 was unable to compete with these designs, and was the only one not ordered into production.
Engine: Two Pratt & Whitney R-2180-S1A1-G radial engines
Power: 1,150hp at 2,350rpm, 1,400hp at 2,500rpm
Empty Weight: 12,760lb
Loaded Weight: 18,230lb
Maximum Speed: 257mph at 5,000ft
Cruising Speed: 200mph
Range: 1,300 miles
Guns: Six .30in machine guns
Bomb load: 2,700lb