Well, in answer to HTE's question, who found this picture -
It was one of about 18 pictures, all from Drew Field,
taken in early June, 1942.
So, we have Drew Field - Check - Air Force was shot around there
We have June, 1942 - Check - Air Force was shot between May and October
It is a B-17B - Check - Mary Ann was a B-17B
The guy wrote in the margin 'The "Mary Ann"' - Check - That was the name of the plane in the movie
The number 15 is on the nose - Oh... - Mary Ann was #10.
Unfortunately, it seems that this is more probably B-17B 39-001,
which was marked #15 in the movie.
However, I have a consolation prize:
I now have John Garfield's signature, found on the back of the picture!