British markings being applied to an RAF Mustang 1a at North American Aviation plant in Los Angeles. At the request of the RAF, North American redesigned the Mustang to be armed with four 20mm in place of the original armament of .30 and .50 caliber machine guns. The new version was given the charge order number NA-91 and 150 were ordered for the RAF through lend lease. When the United States entered the war, 58 of the NA-91s were deferred to the USAAF where they were known as the P-51 Apache. (1942)