Army Air Forces Statistical Digest

If you want data on the USAAF during WW2 then the best source is the ''Army Air Forces Statistical Digest '' 3 vols.It can be found on the site of the Air Force Historical Research Agency  : http://www.afhra.af.mil/timelines/index.asp .

You can also find many interesting studies written after the war by German personnel (they start at  150 )and the best part is it's all free!
Strength data for European and Med theatre of operations are given in tables 88-90 vol 2.
Here is a table i made :