In response to an Air Corps request for twin-engine attack aircraft, the Stearman Co. designed and built the X-100. The Army specifications called for an aircraft capable of carrying 1,200 pounds of bombs for 1,200 miles at a speed of at least 200 mph. Five companies completed design studies: Bell, Douglas, North American, Martin and Stearman. The Bell design never got past the design phase, but the other four designs were built at company expense and submitted to the Army for flight testing. Besides the Stearman X-100, Douglas built its Model 7B, Martin built the Model 167F and North American built its Model NA-40.
The Douglas 7B and North American NA-40 both crashed during testing -- the Douglas aircraft in January 1939 and the North American NA-40 in April of the same year. The Army wasn't satisfied with any of the other aircraft and requested new designs from all five companies. The Army Air Corps eventually bought the Stearman X-100 and designated it XA-21 with serial number 40-191.
The XA-21 was equipped with 1400-hp radial engines and carried a normal crew of three: pilot, bombardier and gunner/radio operator. The plane had nine .30-cal. machine guns for defense and carried a maximum bomb load of 2,700 pounds. The most unusual feature of the XA-21 was the large greenhouse cockpit enclosing both the pilot and bombardier's compartments. This design aided in streamlining but severely restricted the pilot's forward vision by making him look through the bombardier's compartment glass. The Air Corps tested the XA-21 with the original configuration, but ordered a change to a more conventional nose glass arrangement with separate glass for the pilot and bombardier.
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