The North American P-51 Mustang was originally designed in 1940 to meet a specification given by the British. Originally the British Purchasing Commission wanted North American to build P-40s for the Royal Air Force (RAF). However, the president of North American, J. H. "Dutch" Kindelberger, wanted North American to produce a better plane. The British accepted the proposal under the proviso that a prototype be done within 120 days. Within 117 days North American had a prototype prepared by Raymond Rice and Edgar Schmued. It had to borrow the landing gear from an AT-6 and it didn't have an engine.
The original P-51 had an Allison engine which didn't perform well at high altitudes.
The RAF gave Rolls-Royce four Mustangs to have the Merlin 61 engine installed in for testing. This eventually led to Packard building the Merlin under licence.
Part of the reason for the Mustang's high performance was the low drag laminar flow wing.
The Mustang was declared "the most aerodynamically perfect pursuit plane in existence" by the 1944 Truman Senate War Investigating Committee.
Speed and Range Comparison