This is not a definitive history of RAF flying training in the USA during WW2. Far from it! It is best described as a story woven around such training. Accurate statistics are hard to come by. Quantitative data used in this story should be regarded only as an indication of the numbers involved; nevertheless, they are sufficiently accurate to give a good appreciation of whatever they represent.
It is difficult even to be precise about what is meant by the term "RAF aircrew". The WW2 RAF comprised many nationalities some of whom were grouped into separate units for command purposes. One example was No.6 Group of Bomber Command which was manned entirely by Royal Canadian Air Force (RCAF) personnel and this Group comprised one-fifth of Bomber Command! Most of the flying training for the aircrew of No.6 Group would no doubt have been done by RCAF flying schools in Canada which themselves were part of the wider training scheme originally known as the Empire Air Training Scheme (EA TS). Similar arrangements applied to Australians, New Zealanders, South Africans, Southern Rhodesians, Americans, etc.. Therefore the RAF comprised many nationalities but it is probably true to say that the bulk of RAF pilots who trained in the USA in WW2 would have been of British nationality recruited in the United Kingdom through the RAFVR (Volunteer Reserve) scheme.
In passing it is worth mentioning that the Canadians have a historic contribution to British military aviation which is out of all proportion to their population. In WWI 30% of the pilots in the Royal Flying Corps (RFC) were Canadian! The Canadian contribution to Bomber Command in WW2 has already been mentioned and this was out of a population of some18 million people of whom about 6 million were in the French speaking province of Quebec and they were, at best, luke-warm in their support for Britain's war effort, being of the opinion that Britain was the cause of the French collapse in June 1940!
Scholarly Commons Citation
Croisdale, D. (1999). RAF Flying Training USA 1941 -1945 (As written in 1999). , (). Retrieved from https://commons.erau.edu/bfts-history/4