American war production is elastic though. At any given moment, capacity is limited, but the greater the prospects of profitable production are, the more investment there is in new plant.With additional French purchases, along with historic British purchases, there's likely even more constraints on both Curtis and North American..... Who's the most likely candidate(s) to backfill backlogged orders - at that point in time?
As a general thing such expansion has a limited rate. One thing that might have limited the resolve of US investors in opening up yet more aviation production lines would be the thought that their main OTL customer, Great Britain, could achieve peace with Hitler any time HM War Government decided to call for a truce. If you build a whole new assembly line in Kansas or Georgia and then the war in Europe stops, it is just another bad investment made valueless.
OTL US war production ramped up to insanely higher levels once the USA itself entered the war, which put the government in the position of making assurances to the various firms. Constraints continued and always would exist, limiting the number of airframes and other war materiel the US firms could churn out in a given week...but fear that a financial investment in expanding plant might prove to be a costly error is not going to be one of them! If a given firm proves to be incompetent (this happened to both Brewster and Curtis during the war OTL) then the authorities will seize their plant and hand it over to better management, but barring punishment for specifically culpable individuals, the owners are entitled to compensation and will get it.
Now consider that in this ATL we are not in a period where only one government, the British War Cabinet, can single-handedly decide to either continue or end the war. There are two major Allied governments; one is in exile all right, but exiled to substantial holdings with considerable value (to reassure US investors who might have doubts about actually getting paid) and with even more skin in the game than the UK and Commonwealth generally--the idea that the Continent must not be dominated by a single dictator might seem too abstract for Americans to have confidence the British won't throw in the towel and cut their losses, but the very country the entire exilic French Government and military forces purport to defend is under German occupation! The whole premise of the French government in exile is winning the war and taking their homeland back. They might go bankrupt; they are not going to want to quit. So the composite Entente of Britain and France together between them have both credit and resolve.
This is not quite as good as the US government writing the various firms a blank check to cover any war-winning related expenses they might claim at the Congressional pork feeding trough, but it is pretty reassuring for the short run anyway, and so we can expect more production capacity being built up faster than it was OTL in this period, with what was accomplished once the US belatedly entered the war in late '41 being an index of how much faster it might build up versus OTL in this time frame.
Another set of limits that usually applies but does not apply here, in either OTL or TTL--the USA is still emerging from the Depression. There is just lots and lots of slack potential capacity to engage that will not suck up much in the way of scarce assets, certainly not fundamental ones like say labor, even labor with specialized skills. Capital is another story, but then again, to say we are still hung over by the Depression is precisely to say there is capital capacity sitting around idle.
As the USA geared up for total war after Pearl Harbor, limits were hit and various economic reactions took place such as galloping inflation (offset by soaring wage levels and price controls and rationing of goods to be bought with accumulating wages--war bonds became the major form of savings, which closed the circle of US self-financing and in effect mortgaged the entire capital value of the whole USA for a wartime budget. When this happens, the dynamic of US war industries will change and become preoccupied much more with US rather than foreign customers. But at this point, US open involvement is only something FDR is hoping to work his way toward gradually, and rising military procurement on "preparedness" pretexts is not crowding the market too much--it is actually expanding it.
So in this period especially, Fall of France (but here this being a geographical but not political thing) to Pearl Harbor, I think US firms will simply grow more rapidly due to greater investor confidence these investments will pay off, and while I think I cannot argue that none of the conflicts others have brought up will happen, that instead of say the Mustang being cancelled to make room for more P-40s, actually it will be delayed a bit and initially produced in smaller numbers in some new plant that did not exist OTL, and when it is appreciated a bit more, this plant will expand in parallel with the one producing the Warhawk surplus.
What I wonder is whether it is possible that higher revenues might enable Curtiss to pull out of its OTL funk and come back with a successful late war design that finally takes them past the P-40. OTL they offered many, including a long range jet as their last gasp, but none were deemed good enough. Part of this was apparently a stagnant corporate culture. Can heavy Warhawk procurement at this early hour, combined with AdA confidence in the Curtiss brand, lead to something neat?