There were a number of contingent factors that were in alignment that allowed for such a swift development of the NA-73X which eventually became the P-51. Some of these factors were technical in nature, but others related back to human factors like the right business climate and the nature of North American's design team. All of these worked to form a relatively unique constellation to allow for the quick turn around time.

Firstly, the advent of the war and the Roosevelt administration's more permissive attitude towards British tendering offers made aviation firms much more willing to participate in riskier projects. Contracts and money were now available even though the US was still neutral. The wartime milieu helped North American in two key ways. Firstly, it meant that after the lean years of the Depression, firms finally had massive orders and designers the opportunity to work on projects hitherto only consigned to paper. As the Mustang's chief designer Edgar Schmued later recalled, at this time he

had a burning ambition to build the best fighter. I made many sketches without knowing what it might end up as, but when the time came I was ready.

American aviation firms in prior years had to depend on penny packet orders for military contracts, with protracted development cycles and prototypes. This meant that engineers like Schmued were well primed for a more permissive environment.

The second way the war helped North American is that it swamped one of the US's largest aviation concerns, Curtiss. The NA-73X actually grew out of a British desire for Curtiss P-40s, but the Curtiss factories were swamped both with domestic and international orders, so the British initially approached North American head James "Dutch" Kindelberger in January 1940 floating the idea that they could license produce P-40s Curtiss could not. Kindelberger allegedly approached Schmued over the possibility of building P-40s, to which the engineer replied "don't let us build an obsolete airplane, let's build a new one." Both Schmeud and other North American engineers were aware of the various problems Curtiss's somewhat conservative approach towards design had upon the design of the P-40, and they knew that elements of the P-40 like the chin radiator were compromises that ate into the plane's performance.

But neither funding nor eagerness will guarantee an end product and the NA-73X had another element working in its favor: data. Although actual orders for aircraft were slim in the late 1930s and 40s, this was an era of great experimentation and research in aeronautics. This meant that Schmeud and his team had a wealth of data which to base their various designs to meet the British order. This helped the engineers confirm certain design principles or avoid dead ends without having to go though a laborious process of testing. North American managed to buy from Curtiss its data for its follow-up for the P-40, the XP-46, which helped confirm a number of principles behind the Na-73X. Although Curtiss engineers would later claim that North American simply plagiarized the XP-46 data and a number of North American engineers countered that the design was well underway by the time the firm acquired the plans, the data probably helped boost confidence in the project. As North American chief engineer said of the XP-46:

we ran a quick study and said this was just a rehashed P-40, and we didn't see where it's all that great, and we'd do better starting from scratch.

The adaption of a laminar-flow style wing was a little less ambiguous. North American was able to acquire data from the NACA and the Guggenheim Aeronautical Laboratories, California Institute of Technology about low-drag airfoils that was critical in the development of the NA-73X's wing.

Finally, there was also a culture at North American in 1940/41 that was conducive to the sort of high-pressure design work to meet the deadline of 100 days from blueprint to design. North American's engineering teams worked seven-day work weeks with work schedules up to 16 hours. Timing here was crucial as North American was still a small enough firm that there was not a layer of bureaucracy, as existed at Curtiss, that could muck up the design process. When mistakes were made in the process, there was less worry about recrimination and reporting the problem and much more on fixing the problem. Schmued later recalled in 1984:

We could never build another plane today as we did then. Today they just don't have what it takes. There are too many levels of authority within the building companies. They have a president, a vice-president, another vice-president, still another vice-president, and many other levels.

Although Schmued perhaps waxed too nostalgic in his recollections, North American's corporate structure was loose enough to allow the engineers more free reign. This was a period in which many of the heads of aviation firms were themselves once engineers and they understood aspects of the design process as opposed to being concerned with strictly business.

In hindsight, North American's achievement in its 102 day process was not that remarkable. The war witnessed a number of other crash aviation projects that telescoped the design process, such as the He-162. Unlike the ill-fated German Volksjager, North American had the benefit of using preexisting technology and design principles to produce an excellent airframe; the NA-73X was a remarkable aircraft, but it was not bleeding edge technology for 1940. The first flight of the NA-73X also was delayed by 15 days from its 102-day completion because Allison was slow to deliver the V-12 engine. This illustrates another problem that bedeviled aviation designers in this period: bottlenecks. As aircraft became more sophisticated, so to did the satellite industries producing components and equipment for aircraft. North American operated with the relative luxury of not having to worry about its various suppliers producing the necessary equipment. Pace Schmeud's assertion that "we just don't have it in us anymore," it is important to realize that the NA-73X operated in a time when such insular small design teams were soon to become a thing of the past.

Sources

O'Leary, Michael. P-51 Mustang: North American Aviation. London: Osprey, 1998.

Wagner, Ray. Mustang Designer: Edgar Schmued and the P-51. Washington, DC: Smithsonian Institution Press, 2000.