Americans all, they sign up to fly in the RAF, because the United States is not yet at war!
There really was an Eagle Squadron. Americans crossed the border and signed up for the Royal Canadian Air Corps. Charles Sweeney, an American businessman in London, persuaded the British government to form an American squadron in the RAF -- interestingly, his uncle, also named Charles Sweeney, was in France persuading the French government that they needed an American squadron. Thousands of American volunteered, many died in training -- paid for by Sweeney and his friends -- 244 served in the Eagle Corps (along with 16 Britons), and a hundred died in the service. When the US joined, the three active Eagle Squadrons were transferred to the Eighth Air Force of the US Army Air Forces. They maintained some markers from the service in the RAF.
Although this Walter Wanger production doesn't have any first-rank stars -- perhaps deliberately -- he was clearly casting for the long term; Alan Hale Junior shows up early, and so does Diana Barrymore. Although there's nothing in here that you haven't seen in a hundred B pictures about the air forces, director Arthur Lubin was clearly given a budget here, so it's a B movie write very large on the screen and very watchable.
When it was premiered in London, there were two rows of pilots from the Eagle Squadrons in the audience. They left before the end.
