The Bendix ball turret on a B-17 was mounted on a rigid suspension gimbal from above. It was not designed to be retractable. The gunner entered from a hatch on the upper back part of the turret. Unless he was extremely small he did not wear a parachute while in the turret (a Houdini type act to do...). Unless the turret was badly damaged exiting it should have been possible in almost any circumstance (a hand crank was available for loss of power). During take off and landing the ball turret was unoccupied.