Gunners where effective every now and then but not even heavy bomber formations with hundreds of gunners where enough to protect bombers from fighters.
I have a very strong feeling that all those gunners with exceptions of the rear gunner were totally useless, except for the false feeling of protection that they gave the other crew and some "fear effect" towards the attacking LW. Unlike in AH, hitting a fighter flying above or to the side of the bomber means shooting through a 200 mph crosswind at variable angles, variable elevations and variable amount of lead required. They probably could not hit anything from these directions unless it was flying in formation with the bomber. The rear gunner at least had a much smaller cross-wind component, typically were shooting at small elevation angles and at planes coming straight at them (no lead required), so he stood some chance of hitting an aimed shot.
I would love to see some statistic about % of claims (as unreliable as they are) by B17/24 gunners split by gunner position. Or alternatively, some statistic about damage received by LW fighters by aspect of attack. The only hits on fighters that I read about in pilot stories were obtained by (with high probability) the rear gunner, though I must say that I read mostly allied pilot books and LW bombers did not have many side/top/ball gunner shooting in other directions.