If you see a photo of a 17 in flight with the guns pointed straight down with the ball turret, it means the gunner's hatch is inside the plane and the gunner may be inside the fuselage not the ball at the time.
If you watch some of the more well known gun camera footage, you'll see the ball pointing straight down. It's a pretty good sign the gunner has been told to vacate the ball because they are abandoning the plane.
B17 ball was not retactable, B24 ball was. In the Pacific, more often then not they removed the ball and had tunnel guns instead. This was common practice with the 5th Air Force 24s