 On 23 June 1942, a contract was placed for 1,200 P-51As (NA-99s).The P-51A used the new Allison V-1710-81 engine, a development of the V-1710-39, driving a -diameter three-bladed Curtiss-Electric propeller.The armament was changed to four .50 in (12.7 mm) Browning machine guns, two in each wing, with a maximum of 350 rounds per gun (rpg) for the inboard guns and 280 rpg for the outboard.