Don't want to step on Rick's toes, but I was inside a B-17G only two days ago. It was the Collings Foundation "Nine-O-Nine". They were at Paul Allen's Flying Heritage Collection at Paine Field in Snohomish County, Washington.
The inside of that B-17G is really cramped. And really dark. I had my 4x5 Crown with flashbulbs, anticipating only exterior shots. I didn't even try inside because the bulbs would have been way too powerful at those tight distances.
I would think at least a 24mm on a 35mm camera. But if you could go wider, I would. 20mm, or even a rectilinear 18mm or 15mm would not be out of place. There's a lot to show, but not much room to maneuver.
And as I said, it's dark. Not much light, and everything was painted olive drab or brown or black. Inside the fuselage, think of shooting with a flash down a mine tunnel. The nearest walls will be overexposed in order to get the end of the tunnel correct. And side-to-side wall shots will be made at not much more than arm's length. The more film speed the better, I would think, to enhance depth-of-field in the tunnel shots.
I do remember thinking that a synchro-sunlight-balanced bare bulb with a very wide angle lens might have worked well in the cockpit.