5 May 1943: First flight of a production North American Aviation P-51B-1-NA Mustang, 43-12093 (NAA serial number 102-24541).
Following the RAF’s initial disappointing experience with the Mustang I (P-51A), Ronald Harker, a test pilot for Rolls-Royce, suggested fitting a Merlin 61 engine, a two-speed, two-stage, inter-cooled supercharger, designed by Stanley Hooker of Rolls-Royce.
This gave an increase in horsepower from the Allison’s 1,200–1,620 horsepower (890–1,210 kW), or 1,720 horsepower (1,280 kW) in War Emergency Power, delivering an increase of top speed from 390 to 440 mph (340 to 380 kn; 630 to 710 km/h), as well as raising the service ceiling to almost 42,000 feet (13,000 m).