The bombload of a B17 changed alot over the course of the war, as engines made dramatic improvements in power output.
But regarding specifically your comment, all raids into Germany were long range raids - this is why the bombers had no fighter escort until late in the war.
Also, targeting was a huge problem. The norden bombsite was a major US innovation, a highly guarded secret. The US braved the fighter cover to do daylight raids and it was easier to be accurate, while the British, more sensitive to the loss of men and equipment, carried out nighttime raids where the skill of the lead bombers to light up the proper target was paramount to the success of a given raid.
This is a long, long story, probably best left to another message board.
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