You can knock out gunners. I was flying a B17 the other night and took 3 attacks by a P51... lost my top and bottom ball turrets, and my 2 waist gunners. Also lost 3 engines, some fuel tanks, an aileron, and something else I forgot. Talk about being a little nervous while limping back to base.
Still managed to have my tail gunner and nose gunner survive though. The tail gunner actually managed to take a piece out of the P51 which ran home... as did I.
Soda