How long did it take to design the P 51 Mustang fighter plane?

In direct response to a requirement order from the British Air purchasing Commission in early 1940, plans were underway for the NA-73X in early April 1940. The prototype flew on 26 October 1940 about 150 days later. It showed Excellent performance and the RAF ordered 320. All had the Allison F3R engine and were used as tactical recce aircraft. They had 4x.50 and 4x .30 cal machine guns. North American built 2 extra and sent them to the USAAF selection centre as the XP-51. The USAAF then ordered 310 as sent, and a further 500 with the more powerful Allison V-1710 engine.