Nemisis, I don't think its got anything to do with the bombers we have in the game. I think its just you.
I fly B-24s more than any other bird in the MA and I can honestly say that in a liberator, I have never been shot down while above 20k. Though I've had plenty try, the guns are more than sufficient to take down the occasional interceptor and even after sustaining significant damage in some cases, I always make it to my target, roll my bombs off and return home every time.