In 1940 British purchasing agents asked North American Aviation to build more Curtiss P-40s. The P-40 was the only American land-based fighter available at the time but it was also seriously deficient in speed, range, and altitude performance. North American declined to build it but proposed a completely original design and promised to finish it in less than three months. North American completed the NA-73 airframe 117 days later and installed an in-line, liquid-cooled Allison engine. This aircraft first flew on October 25, 1940. During flight tests, the airplane exhibited outstanding performance, particularly in level-flight speed. The British enthusiastically ordered 150 examples of the airplane and the Royal Air Force (RAF) named it the Mustang. The U. S. Army Air Corps also showed interest and in 1941 they ordered 500 ground-attack variants designated the A-36.Early in 1942, the British tested four RAF Mustangs fitted with the two-stage, two-speed Merlin 65 engine. The supercharger in the Merlin 65 was optimized to produce sea-level horsepower up to approximately 9,150 m (30,000 ft). Urged on by Rolls Royce and British and American fighter pilots who sampled the new Mustang, North American obtained two Merlin 61 engines similar to the 65 and installed them in airframes designated XP-51Bs. The new variant had good range and outstanding speed, about 80 kph (50 mph) faster than previous models. Thus was born the world