Once the plane got off the ground and into the air...and the navigator got the plane to its desired position...and the bombardier got their bomb load to the target...the coming and going often depended upon the aerial gunner. On the B-17 "Flying Fortress" bomber there were up to 13 -50 caliber machine guns. [Lucky number I guess.]
Upper turret, belly turret, nose turret, and tail gunner all had their assigned guns. During an air battle, all crew members (except the pilots) joined the fighting. A bullet with wings it is. Fire at will.