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.
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.