I vaguely recall reading that the ball-turret was (counter intuitively) the “safest”, or nearly so, position in the B17 or B24. It presented a relatively small target, and the plexiglass is about 2” thick.
The gunner I saw mentioned those things and added that the two .50 caliber machine guns were big chunks of steel that also offered some protection from flak. That said, nothing on a B-17 could stop a 20mm cannon shell or a direct hit from an 88, so everyone on board faced the same threats.
There was no safe place on an unarmored airplane.
Just an 8mm ball round could go in one side of the plane and out the other. Too bad if you happened to be in the way.