While North American (NAA) had developed the prototype quickly, the first stage of production moved along more slowly. The first NA-73 production aircraft did not fly until April 23, 1941, six months after NA-73X. It carried no weapons and was kept by NAA for testing and development. The second production airplane (armed with four .30's and four .50's) arrived in Liverpool in October, 1941 - a year after the prototype's first flight.
Nonetheless, the Mustang was so promising that in late 1941 the RAF ordered another 300 and the USAAF 150. As the exigencies of war demanded, 93 of these 150 (factory designated NA-91) ended up in British service, as Mustang IA's, equipped with four 20mm cannon. The remaining 57, equipped with four .50 caliber machine guns, and known as P-51's, ended up in US service.