I find I can amputate a wing of a B-17 reasonably often with a P-51 (or any other 6x50 or 8x50 cal. aircraft) in one pass firing at the wing root when my gunnery is fairly good, which isn't nearly often enough.
Given what we know about B-17 wing strength, I suspect that's pretty dubious; B-24s maybe.
However, the other posters are correct; buffs are already getting so many unrealistic concessions to gameplay (mind you, I don't have a problem with these) such as laser-guided bombs, gunners that actually hit attacking planes (relatively, very, very few LW aircraft were actually shot down by B-17s; it was overwhelmingly the escorts) that it's kind of a chutzpah thing to whine about durability :-)
Realistically, a lone B-17 should probably take several more passes to kill than it does right now, but it should be far easier to kill a lone B-17 in a fighter without taking a ping.