As I remember, most of the B-17s were being sent to the Pacific at that time. The thinking seems to have been send the bulk of the Navy to the Atlantic, Keep a smaller naval force in the Pacific and support it with more air power.
Most of how many? See below for details.
Thats great, so they can launch their operations from their home fields in USA? Building airfields in UK was a massive civil engineering project using up to 60,000 people, I can't see that this civil engineering was ever considered pre war. How many operatioanal squadrons of heavy bombers did the USA have on Sept 3rd 1939?
First flight of the B-17B (the first operational variant) was on 21 June 1939. A whopping 39 B-17Bs were built. Even if we add all those to the totals of prototype variants, we get no more than 53 airframes in the entire US inventory. Of course, I very much doubt that all 39 B-17Bs were built between 21 June and 3 September 1939...but it's possible. However, there wasn't a substantive B-17 force in the USAAF inventory in 1939.
The B-24A didn't arrive on the scene until the summer of 1941 so we can completely exclude that from consideration in 1939.
From what I can find, the B-17Bs equipped the 7th and 19th BGs based at Hamilton Field and March Field respectively (both in California) in 1939.