My father’s B-17G Field Service Manual shows three 24 volt, 34 Amp hour batteries. Number 1 is in the leading edge of the left wing inboard of # 2 engine, and numbers 3 and 4 are in the leading edge inboard of engine # 3. There is no battery # 2; perhaps it was in earlier airplanes?
For what it is worth each engine had a 200 amp generator.
The APU appears to have been for ground use only. Also some were not permantly installed, these had to be removed from the airplane and run on the ground as a GPU.