The ball turret gunner had it extremely rough. When I was a kid, I had a chance to meet a ball turret gunner at an airshow. He was on B-24s. I was looking at the ball turret of a B-17 (I believe the gunners hatch was open and I was peering inside of the turret) and this old man walked up and asked if I minded if he looked. He told me he used to be a ball turret gunner. I wanted to ask him how many kills he got, but I figured that was rude, so I asked him what year did he serve. He said 1943. Wow. B17s and B24s were offered up for the slaughter in 42 and 43. Anyhow, it was very cool meeting him.
The ironic part of this is the B-17 was designed with a remotely belly turret. However, the main reason it was removed is that during trials, it was said to induce nausea and airsickness in the gunners when sighting through the periscope. If you puke in your oxygen mask at 25,000 feet, you're as good as dead due to it freezing and blocking airflow.
