Against the targets flying at 15000 ft, the accuracy of 88s is maybe an order of magnitude greater than vs. targets flying at 25000 ft. They will spend less shells per hit or damaged bomber, meaning barrels will be less worn. So the guns will remain accurate for longer time. The time gun has against target jumps, again, by an order of magnitude, it can also engage the bombers that are moving away from the gun. The bomber flying at 15000 ft will be slower than one flying at 25000, again making the gun crew to have better aim on them, and providing even more time. The defending fighters will spend less time fuel per kill, with less time spent to return in fray once rearmed refueled. The bomber crews showered with Flak fragments will be anything but efficient.
Now this an interesting thought. Low altitude bombing, fighter escort could stay low as well. No P38 high altitude problems, Allison P51's could work, P40's could work here.
But I doubt a slow, heavy 4 engine bomber would work, at low altitudes, in daytime, over Germany for the reasons above.
If no B17's in 1939, there would have been another 4 engine heavy in use. Considering the bomber crowd.