They didn't fly with them though. You'll note in the diagram that's an early model with open windows and wind deflectors?
Those were done away with. You had the windows closed in by plexiglass, you had the wind deflectors removed because of that. You had the radio room gun removed entirely. It wasn't doing any good, had a narrow range of focus, and (guessing) was probably distracting for the radio operator when he had better things to do in the middle of combat.
"Up until the later models of the B-17G, the radio operator had a .50 caliber machine gun located in the ceiling of his compartment that faced the rear of the aircraft. It was found that this was the least effective gun position on the aircraft since the majority of fighter attacks came from the front. Once fighter activity lessened during the latter half of the war, this gun was removed."
http://www.b17queenofthesky.com/positions/radio.php
