North American XB - 21 The North American XB - 21 , also known by the 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 Twin Hornet radial engines , which were fitted with turbosupercharger s 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 gun s . 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 , company test flying indicated a number of minor problems . Modifications resolving these resulted in the aircraft being re - designated NA - 39 , and , accepted by the U . S . 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 the fly - off , 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 $ 64 , 000 USD , while North American ' s estimate was $ 122 , 000 USD per aircraft , and an order was placed for 177 of the Douglas aircraft , to be designated B - 18A . Despite this , the Army Air Corps found the performance of the XB - 21 to have been favorable enough to order five pre - production aircraft , to be designated YB - 21 . However , soon after this contract was awarded , it was cancelled , and none of the YB - 21s were ever built , leaving the XB - 21 as the sole example of the type ever constructed . Operated by North American Aviation , the XB - 21 served as a research aircraft until its retirement . Legacy . Although the XB - 21 failed to win a production contract , it was the first of a long line of North American Aviation medium bomber aircraft , and provided experience and knowledge that assisted in the development of the North American NA - 40 , which , developed into the B - 25 Mitchell , would become one of the Army ' s standard medium bombers of World War II .