Doing a little more research I'm pretty sure that it's not a flight jacket of any sort, reason being is that it's too long. Flight jackets are almost cut short, ending above the hips, to make it easier for the wearer to sit while wearing. What I
think it might be is a modified version of something called a duffel jacket, however, every example of dufflel jacket that I've seen so far has a hood, which Steve's does not. Failing finding an exact match, you could but one that's close and either cut off the hood and add your own collar or mod the hood into a collar.
Edit: I just found this:
https://www.filmsjackets.com/steve-trevor-coat Hope that helps.