Old planes were much more “airworthy” than modern ones, less dependent on the power of the engine to keep them aloft.
The flip side of saying they didn't have as powerful engines.
I remain highly impressed with what they did. Yes, the B-17's bomb bay was surprisingly small (to me), but after looking at the airframe those bombs had to be a rather significant portion of the plane's weight, and the designers took all of that into account.
I look at this with an engineer's eye (even though I may not be skilled in the particular disciplines involved) and I remain impressed.
WWII resulted in much evil, evil not reduced by the rapid technological advances it spurred, but we can at least benefit from those advances. (The medical knowledge acquired from the Nazis is a different issue.)