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I think that the waist gunners were the most exposed. Not much separating them from the flak and enemy shells being fired at the plane, except any body armor they wore, and any flak jackets they may have lined the floor with. (A fairly common practice.) Except for that, just some thin Alclad.
Reading the information on the B-17 gun turrets got me thinking. Were there any specific gun positions on bombers that were more hazardous than others? That is to say it may have been more hazardous due to enemy fighter tactics, exposure to enemy fire (less assisting coverage from other guns), etc. For example, it sounds like the belly turret of the B-17 was a real "sorry about your luck, buddy" position, but was it more dangerous than any other gun position?
As a side question: how did they select which gunner went to which position?
Feel free to comment on any bomber allied or axis.
Also it was extremely hard for the gunners to manuveur even one 50 cal in a 175 mph slip stream. It required alot of strength. I can't imagine it would be an easier with two 50 cals.