The first generation of new, monoplane bombers (i.e. Wellington and Whitley) had the "dustbin" type of turret which was lowered into the slipstream when required. A single .303"(*) and a loss in performance due to drag made them unpopular. I certainly wouldn't have wanted to go to war in one of them!
The second generation of Bombers, Manchester, Halifax, and Stirling had more advanced turrets where only a small proportion of the turret projected below the fuselage. This meant the gunner sat inside the fuselage and sighted through a periscopic sight. These turrets were considered useless at night due to the loss of vision through the sight and a tendency for the gunner to quickly become disorientated when traversing.
(*) I was thinking of the very early dustbin turret in the Heyford when I typed this. The later, retractable turrets in the Manchester, etc, all had two .303" Brownings.
Last edited by 682al; 18th Aug 2017 at 11:19.
Reason: Typed in too much of a rush!