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I've read where in a few cases the B-17 hydraulics were so heavily damaged, the ball turret was jammed and landing gear stuck in the retracted position. The pilots had to "belly land" knowing it would kill the ball gunner, but there was no other option to save the rest of the crew.
It took big stones to crew in the power turret on the belly. The turret was armored, but their life expectancy was low. There you are in a fetal position on the bottom of a bomber at 35,000 ft with FLAK and fighters...big stones.
Good documentary here---https://www.youtube.com/watch?v=rqypIedychkGunners trapped in the turret were lost as well in ditchings in the sea.I've read that you could always spot the ball turret gunners at reunions. They were hunched over because their kidneys were shot from staying in the turret for so long. They didn't want to risk leaving the underside of the bomber un-protected.In my dad's group, one of the vets recalled that on one mission they recorded the lowest air temp at altitude for the war (for them) something like -65F. He said that their ball turret gunner on that mission lost his feet from frostbite.
Last summer I got to fly in a B-17 out of Willow Run Airport to honor the father of some family friends. Those planes might look fairly large on the outside, but all positions were cramped on the inside. I don't think the tail gunners get enough credit for having to perform where they did. They actually had to sit on a bicycle type seat in a kneeling position for hours on end.