I doubt there were gun cameras installed on B-17s because:

1. I've seen a lot of WW2 aviation footage, and none appeared to be from B-17s.

2. The combat footage that exists for B-17s was shot(!) for the "Memphis Belle" documentary, and some footage for it left on the cutting room floor. This was filmed by a camera crew going along for the ride on several missions. Memphis Belle footage appears in countless WW2 documentaries.

3. My dad flew 32 missions in one and never mentioned gun cameras, though he did talk about them when he flew ground attack missions in the Korean War. I have a couple of them from his airplane that the military didn't want.