As I indicated earlier, your Grandfather served with Eighth Air Force, a major component of U. S. Army Air Forces (the U. S. Air Force didn't become a separate branch of service until 1947). Eighth Air Force was (and is) nicknamed "Mighty Eighth". Contact the Mighty Eighth Historical Museum at their website < http://www.mighty8thmuseum.com/http://www.mighty8thmuseum.com/ > (click on address to go there). They have an extensive collection of reports, rosters, etc., and also have a message board. I'm informed they now charge a research fee to non-members but you can check that with "the powers that be". They may be able assist in establishing his squadron and/or group and may even be able to put you in touch with individuals who served with him.
Another website, < http://www.heavybombers.com/http://www.heavybombers.com/ >, may be a source of assistance. If all this fails, send me the info I suggested and I'll locate records.