How long did North American take to get the Mustang prototype ready?

The Mustang's timeline begins in spring 1940. The British Purchasing Commission approved the detailed design drawings, and the project formally began on 4 May 1940; the firm order for 320 aircraft followed on 29 May. Only a few weeks separate the two dates, and that double reference explains why the prototype's timeline admits more than one reading.

The NA-73X left the workshop in September 1940, 102 days after the order was placed. That is the figure most often cited to measure the program's speed. The aircraft did not fly, however, until 26 October 1940: between the roll-out and the first flight lay the usual fine-tuning work of any prototype. With test pilot Vance Breese at the controls, the aircraft handled well and demonstrated it could carry a notable fuel load, the trait that would come to define its career as a long-range escort fighter.

The second figure in circulation, 117 days, does not contradict the first: it changes the starting point. The 102 days are counted from the British order; the 117, from the moment the prototype's construction was authorized. Depending on which of the two milestones is taken as day zero, the same interval to the September 1940 roll-out yields a different number, and both figures coexist in the literature on the program.