North American P-51 Mustang XP-51B

Discussions between Major Thomas Hitchcock, Rolls-Royce engineers and North American, together with the British work on the Mustang X, led in July 1942 to a contract for two North American Merlin prototypes, briefly designated XP-78 and soon renamed XP-51B. The airframes chosen were the 93rd and 102nd from the NA-91 order, set aside for the purpose as early as mid-1941. Working from the Packard V-1650-3, which duplicated the Merlin 61's performance, North American estimated a top speed of 445 mph (716 km/h) at 28,000 ft (8,500 m) and a service ceiling of 42,000 ft (13,000 m).

Unlike the British Mustang X, the two XP-51Bs were a thorough conversion: a tailor-made engine installation and a complete redesign of the radiator duct. The structure was strengthened, the fuselage and engine mount area receiving extra formers, because the Packard V-1650-3 was 355 lb (161 kg) heavier than the Allison; the cowling was completely redesigned to house it, since the intercooler radiator mounted on the supercharger casing made the engine 5 in (130 mm) taller and its carburettor was of the updraft rather than the downdraft type. The new fuselage duct held a larger radiator incorporating a section for the supercharger coolant, with an oil cooler ahead of it and slightly lower in a secondary duct fed from the main opening and exhausting through its own exit flap.

The first XP-51B flew on 30 November 1942. Testing confirmed the fighter's potential: the service ceiling was raised by 10,000 ft (3,000 m) and top speed improved by 50 mph (80 km/h) at 30,000 ft (9,100 m). Army Air Forces interest was such that an initial contract for 400 aircraft had already been placed three months earlier, in August 1942, and P-51B production began at Inglewood in 1943. A duct rumble heard by pilots of the prototype prompted a full-scale test in the 16-foot wind tunnel at the NACA's Ames Aeronautical Laboratory, where it was found to disappear once the gap between the lower wing surface and the upper lip of the cooling duct was increased from 1 to 2 in (25 to 51 mm). The material consulted does not detail the prototypes' armament; the production P-51B retained that of the P-51A.