The Army Air Corps requested design proposals from five aircraft manufacturers for a twin-engine light attack bomber in the late 1930s. Stearman submitted its model X-100. The Air Corps did not order any production aircraft, but did buy the prototype and designate it XA-21. The aircraft originally had a streamlined greenhouse nose compartment covering both the pilot and bombardier's positions; however, the forward vision of the pilot was restricted by the arrangement and the Air Corps requested a change to a more conventional nose. Stearman, which by this time had become a division of Boeing, modified the aircraft and returned it to Wright Field for further flight testing.
The top speed of the XA-21 was slightly reduced with the new nose, but the overall performance of the aircraft was roughly the same as with the original nose. Although the Stearman XA-21 wasn't ordered into production, some of its design competitors had better luck. The North American NA-40 eventually evolved into the B-25 Mitchell. The Martin Model 167F wasn't ordered by the Army, but was successful as an export attack aircraft becoming the Maryland when in service with the Royal Air Force. The Douglas Model 7B became the A-20.
Attack bomber prototype
TECHNICAL NOTES: Armament: Provisions for four wing-mounted .30-cal. machine guns, one nose-mounted .30-cal. machine gun and four rear-firing .30-cal. machine guns (dorsal, left waist, right waist and ventral) plus 2,700 lbs. of bombs Engines: Two Pratt & Whitney R-2180-7 radials of 1,400 hp each at takeoff power Maximum speed: 257 mph Cruising speed: 232 mph Combat range: 720 miles with 1,200 lbs. of bombs Maximum range: 1,500 miles Service ceiling: 20,000 ft. Span: 65 ft. 0 in. Length: 53 ft. 1 in.
Height: 14 ft. 2 in. Weight: 18,230 lbs. gross weight Crew: Three (pilot, bombardier, and radio operator/gunner) Serial number: 40-141