 Due to the specialised nature of increasingly advanced turrets, these were treated as ancillary equipment, being designed and supplied independently and replacing Vickers' own turrets developed for the aircraft.The turrets initially used a Nash & Thompson control unit, while each position was equipped with a pair of .303 in (7.7 mm) Browning machine guns.On many Wellington variants, the Vickers-built ventral turret of the Mk I was replaced by a Nash & Thompson-built counterpart as standard.