Just to set things straight, neither this dio nor Shep's were of a B-17 crash-landing... The dios were of a B-17 that suffered a right-main failure while taxiing, which caused it to swerve off the taxi-way and into the grass..... That's why there is little or no damage to the ball-turret... B-17s generally taxied with only the outboard engines running...
The Sperry Ball-turret was retractable in the B-24, however neither the the B-24 nor the B-17 turrets were "jettisonable", so in a crash-landing or ditching, they stayed in the aircraft... ( The B-24 was a notoriously "bad-ditching" aircraft, BTW, because of the fragile nose and high-wing design.)
It was only manned once the aircraft entered hostile airspace or upon command.. The turret was rotated by hand so the guns pointed down during ingress and egress, and the gunner sat with his back against the closed hatch....The rest of the time the gunner was stationed inside the fuselage... His seat was located in the radio compartment where he sat during T/O and Landings.