Late war B-17s carried a tool kit on the ball turret mount that contained everything needed to jettison the ball turret. There are several stories of B-17s that belly landed without the ball turret that returned to the skies in just a few days. If the ball turret stays in a belly landing, like Glider pointed out, it would likely drive the mount through the top of the aircraft and that would make it a write-off.
Henk, I wasn't trying to sound critical of your posts. I was just pointing out that there are some things that you picked up that are not correct. It's all a learning experience. It was just some friendly advice.