Chin turrents weren't intoduced until the G models, though I believe a few of the F models were retrofitted. All but certain these are 'G' model aircraft, though what series I couldn't begin to guess. The color of the top of the rudder may make the plane in the background look a bit off. The finflash 'S' is curious to me; could be a bird from the 493d BG, but there's not a lot of information available in that picture. Could also be the 34th, but the 'S' doesn't look right for that. Hard for me to tell. Somebody better read on the subject could probably figure it out. Just about has to be the ETO, though, as I don't think G-model B-17s were used in the PTO in any great numbers (mostly B-24s and -29s there, range issues) and the MTO only had six groups, all of those marked with a 'Y' looking thing on the top of of the rudder. Obviously, there were other users out there, but none I think would match up here.
It is a cool picture, though,