The most successful American fighter of World War II was designed for a foreign government and had a conventional appearance. It resembled the Messerschmitt, Spitfire, Yak, and other single-seat low-wing monoplanes powered by inline engines, but its superiority represented the highest refinement of the fighter layout introduced in 1931 by
North American Aviation, at Inglewood, California, had so favorably impressed Britain with its efficiency building Harvard trainers for the Royal Air Force that company president J. H. Kindelberger was asked to produce Curtiss P-40s in his factory. Instead, Kindelberger offered to build fighters designed by his own company.
German-born Chief Designer Edgar Schmued, who had done the NA-50, was by March 15, 1940, ordered to prepare a fighter proposal built around the Allison engine ordered for the Curtiss planes. The Anglo-French Purchasing Commission provided a letter of intent on April 11 for production in 1941 and told North American Vice-president Lee Atwood to obtain current fighter data from Curtiss.*
On April 24 the specification for Model NA-73 was ready and the company ordered construction of a prototype. An offer was made on May 1, 1940, to the Purchasing Commission to build 320 by September 30, 1941. The Army Air Corps granted permission on May 4 for the NA-73 sale, provided it was furnished two airframes, “without charge”. The British ordered 320 on May 29, added 300 more on September 24, and named the fighter Mustang I.
The carefully-streamlined NA-73 was designed for mass production with a low square-cut wing that introduced the modified laminar-flow airfoil developed by NACA to reduce drag. The radiator scoop was streamlined into the fuselage’s underside behind the pilot.
An NA-73X prototype was completed by September 9, but had to wait for the engine, the first 1,150-hp V-1710-39 originally ordered for the P-46/P-40D program. Flight tests were begun by Vance Breese on October 26, 1940, but were interrupted by an accident November 20, 1940.
The first Mustang I production NA-73 was flown April 23, 1941, and was retained by the company for the necessary testing. Next to fly, on May 20, was the first XP-51 for the Air Corps. It was flown to Wright Field on August 24, while the second XP-51, actually the tenth NA-73, arrived December 16. Similar to the RAF Mustangs, they were equipped with armor, leakproof tanks, two .50-caliber guns with 200 rpg low in the nose, and two more in the wings between four .30-caliber guns with 500 rpg.
These features, plus turbosuperchargers, were also specified for the future Air Corps B-24C/D versions, by a contract change dated June 24, 1940. Delivery on the B-24s ordered in 1939 was deferred by the Air Corps announcement on November 9, 1940, that 26 would go to Britain, and be replaced later by modernized B-24C/D models. The RAF aircraft would be delivered with commercial R-1830-S3C4G engines and without armament.
The second RAF Mustang first flew July 3, was accepted in August and began a long journey by ship through the Panama Canal to Liverpool, arriving October 24. By the end of 1941, 138 Mustangs had been accepted.
Tests soon showed the Mustang “certainly the best American fighter that has so far reached this country;” superior to the Kittyhawk, Airacobra, and Spitfire in both speed and maneuverability at low altitudes. RAF Mustangs were issued to Army Cooperation Squadrons, to replace Tomahawks, and were fitted with a camera facing left behind the pilot. Twenty Mustangs were lost on ships on the way to Britain, and four were sent to the Soviet Union in May 1942.
Tactical reconnaissance calls for avoidance of enemy fighters, and so the Mustang’s combat virtues were not yet often demonstrated. No. 26 Squadron flew its first sortie across the Channel on May 10, 1942, and the Mustang’s first air battles were fought on August 19. On October 16, the Allison-powered Mustang was the first single-seat fighter from England to penetrate German air space, and 15 squadrons served the RAF in January 1943, with five remaining in June 1944.
A Lend-lease contract approved September 25. 1941, added 150 designated P-51s to the schedule, and these aircraft were armed with four 20 mm guns in the wings belt-fed with 125 rpg. The first P-51 was flown May 20, 1942, by Bob Chilton, and the first 20 P-51s to follow the last Mustang Is off the line in July 1942 were taken by the AAF and fitted with two cameras for tactical recon work.
This worked so well that they were sent to the 68th Observation Group, whose 154th squadron flew the first AAF P-51-2-NA mission on April 9, 1943. When this contract was completed in September, the RAF had gotten 93 as the Mustang IA, while the AAF got 55 P-51 photographic types (later redesignated F-6A) and two airframes were diverted to the XP-78 project.
Mustang production might have ended in 1942, but Air Force interest in the type was aroused when design began on the NA-97 dive-bomber version on April 16, 1942. Prospects of an improved engine and under wing racks for drop tanks or bombs convinced the AAF to contract for 500 A-36As on August 21, and 1,200 P-51As on August 24, 1942.
From October 1942 to March 1943, North American delivered the dive-bomber Mustangs designated A-36A, whose service was described in Chapter 13 on attack planes. They differed from the P-51 in their V-1710-87 boosted for low altitude operations, dive-brakes, bomb racks, and six-gun armament.