Long time now. Before the B-25 even. I think it was maybe back as far as when we got the B-24, but not sure on exact timeline.
EDIT: I think personally the size of the gunners is too small. I've sprayed hundreds of rounds into the area of gunners (through the bomber fuselage into that area a gunner would be standing) and not disabled the gun or the gunner before. I'd like to see more bomber gunner wounds. I find it very hard to believe I can pump rounds into a tail gunner on, say, a B-24 or B-26 and not either disable the guns or hit the gunner behind them. Seems the bullets go nowhere.