While the early Allison powered Mustangs had excellent low altitude performance, the same could not be said at higher altitudes. In Britain and American efforts were made to combine the Mustang airframe with the Rolls Royce Merlin engine. In Britain that produced the Mustang X, powered by the Rolls Royce Merlin 65 engine, which first flew in October 1942. In America the license built Packard Merlin was used, combined with a two speed two stage supercharger of 1,400 hp, giving in a top speed of 430 mph at 25,000 feet. The XP-51B first flew on 30 November 1942.
The new aircraft outperformed most other fighters above 15,000 feet (although the Allison powered versions were faster below that height). 1,988 P-51Bs were built at North American’s California factory. A second factory was opened in Dallas, where the same aircraft was produced as the P-51C. Both types were given the designation Mustang III by the RAF.
Early P-51Bs were powered by the Packard Merlin V-1650-3, later replaced by the V-1650-7. The aircraft was armed with four .50 inch machine guns in the wings. A fuselage fuel tank was added during the production run, extending the range of the aircraft but causing problems with stability that reduced the usefulness of the extra fuel.
The first unit to receive the P-51B was the 354th Fighter Group, in November 1943. It flew its first mission, a fighter sweep over France, in the following month. By the end of the December it had flown escort missions as far as Kiel, 480 miles inside Germany. On 4 March 1944, the 4th Fighter Group flew the first P-51B/C escort mission to Berlin.
The P-51B was also built at a new North American factory in Dallas, Texas, as the P-51C. In RAF service it was known as the Mustang III. When used as a tactical reconnaissance aircraft the P-51B was designated F-6C-NA. 1,988 P-51Bs were built, with another 1,750 P-51Cs built at Dallas.
Engine: Packer Merlin V-1650-3
Horsepower: 1,380 take-of, 1,600 War Emergency
Max Speed: 440 mph at 25,000 ft
Combat Range: 900 miles with drop tanks
Ceiling: 42,000 feet
Span: 37 feet
Length 32 feet 3 inches
Armament: Four .50 inch machine guns