On April 10, 1940, the proposal was accepted, and the prototype, assigned North American's model number NA-73, was ordered. Just in time for the fall of
France and England's lone stance against Germany. A small number of Spitfires and Hurricanes would begin the defense of England against the Luftwaffe in the
Battle of Britain. Britain's need for fighters was never more desperate.
First Lieutenant Benjamin S. Kelsey. Lt. Kelsey was head of the Army Air Corps Pursuit Projects Office at Wright Field, and he was the single most important
man in the acquisition of what would eventually become the P51 Mustang fighter. With a degree in aeronautical engineering from M.I.T., Kelsey was qualified in
aircraft design and performance, and he ingeniously found the means and the money to keep the program going until America's entry into the war ensured its success.
Kelsey knew of the National Advisory Committee on Aeronautics' (the forerunner to NASA) studies on the laminar flow airfoil, and as an aeronautical engineer,
he understood its importance to new aircraft design and performance. As a result, NACA's Eastman Jacobs was assigned to North American's team that was working on
the new fighter for the British. Raymond H. Rice was North American's chief engineer, Edgar Schmued their chief of design, and Ed Horkey was the aerodynamicist.
Along with Jacobs, they worked day and night, seven days a week, to produce the new fighter as quickly as possible.
Curtiss had been ordered to turn over its design studies and other pertinent information on the XP-46. This included the radiator scoop originally intended
for installation under the fuselage of the P-40. This scoop, which provided cooling air for glycol and oil cooling, was also to have a hot air exit ramp which
would create thrust that more than offset the drag caused by the frontal cross-section for the scoop. Though never fitted operationally to the P-40, it held
promise and was one of the features incorporated in the design of the NA-73. Just how much the data from Curtiss was used is subject to debate. Curtiss engineers
state that it was almost total, while those at North American claim that little of the information was used. The truth is probably somewhere between these two
extremes. Clearly, the NA-73 had a lot in common with the XP-46, and a rational analysis would indicate that the NA-73 could not have been engineered in such a
short period of time without considerable use of the Curtiss data. But equally as clear is the original thinking added by the North American design team. Among
the most important changes was the addition of the laminar flow wing.