The North American XB-21 (manufacturer's model designation NA-21) and sometimes referred to by the name "Dragon", was a prototype bomber aircraft developed by North American Aviation in the late 1930s, for evaluation by the United States Army Air Corps. Evaluated against the Douglas B-18 Bolo, it was found to be considerably more expensive than the rival aircraft, and despite the ordering of a small number of evaluation aircraft, only the prototype was ever built. Design and development North American Aviation's first twin-engined military aircraft, the NA-21 prototype was constructed at North American's factory in Inglewood, California, where work on the aircraft began in early 1936. The NA-21 was a mid-wing monoplane of all-metal construction, powered by two Pratt & Whitney R-2180-A Twin Hornet radial engines, which were fitted with turbosuperchargers for increased high-altitude performance. Flown by a crew of six to eight men, the XB-21 featured a remarkably strong defensive armament for the time, including as many as five .30-calibre M1919 machine guns. These were planned to be fitted in hydraulically powered nose and dorsal turrets, in addition to manually operated weapons installed in waist and ventral positions. Up to of bombs could be carried in an internal bomb bay, with of bombs being able to be carried over a range of . Testing and evaluation Undertaking its maiden flight on 22 December 1936 at Mines Field in Los Angeles, test flights indicated a number of minor problems. Modifications resolving these resulted in the aircraft being re-designated NA-39, and, accepted by the US Army Air Corps as the XB-21. The aircraft, which had been assigned the serial number 38-485, was evaluated early the following year in competition against a similar design by Douglas Aircraft, an improved version of the company's successful B-18 Bolo. During the course of flight testing, the gun turrets proved troublesome, their drive motors proving to be underpowered, and issues with wind blast through the gun slots were also encountered. As a result of these problems, the XB-21's nose turret was faired over, while the dorsal turret was removed. The XB-21 proved to have superior performance over its competitor, but price became the primary factor distinguishing the Bolo and the XB-21. On this account, the modified B-18 was declared the winner of the competition, Douglas quoting a price per aircraft of US$64,000, while North American's estimate was US$122,000 per aircraft, and an order was placed for 177