What makes you think they did? One study that I saw, which was done shortly after the war, estimated that 40% of the damage to US bombers was friendly fire or damage from spent shell casings. The aluminum skins of the bombers was not much defense from a fired 50 cal casing when hit at a 250 MPH. Not to mention damage to props or engines. We quickly learned to put limiters on the turrets to keep gunners from shooting down their own aircraft while tracking an enemy aircraft. .