in the B-17 the ball turret was separated from the rest of the aircraft and only the intercom allowed the gunner to have any contact with the others. it could not open from the inside during flight and it was too small to fit a parachute so yeah, in case something went wrong the gunner was F'd. in the B-24 the turret could be retracted into the aircraft so even if there was a problem they could retrieve the gunner. it still couldn't open from the inside but worst case scenario, they could release the hydraulics and the weight of the guns would make the turret drop bringing the hatch up so that the others could open it and the gunner could exit into the plane