 The 150 NA-91s were designated P-51 by the newly formed USAAF and were initially named Apache, although this was soon dropped and the RAF name, Mustang, adopted instead.The USAAF did not like the mixed armament of the British Mustang Is and instead adopted an armament of four long-barrelled 20 mm (.79 in) Hispano Mk II cannon, deleting the .50 cal "nostril"-mounted weapons.The British designated this model as Mustang Mk IA, and would fit a number with similar equipment.