I just reviewed our correspondence and do not find that I specifically said that he flew on B-17s.The 15th USAAF had Groups that flew B-17s AND B-24s and all manner of light and medium bombers too! (A-36s, DB-7s; B-25s, B-26s etc...)
Until we determine the number of his GROUP we can not say that he flew on B-17s.
Don't get off on the wrong track!
Have you discovered something that specifies his unit besides the patches on the jacket?