The Mk 1 Halifax had two .303s in a nose turret.
It was deleted in later variants as part of the search for improved performance.
According to wiki, a Lancaster Bomb Aimer just had to stand up to man the front turret.
The drawback to the American policy of extra crew, guns and ammunition was it was at the expense of bombload.
On a mission to Berlin, the B.17 only carried about 500lb (or 50lb, depending on whose book you read) more than a Mosquito.