This turret was one of the first fully-powered machine gun turret designs used to equip American aircraft and was used initially on the B-17E Flying Fortress. It is operated by an electro-hydraulic system, which powers both elevation and traverse of the turret's two .50-cal. M2 Browning machine guns, and has a rate of fire of 1,400-1,600 rounds per minute. Turrets of this and later types gradually replaced manually-operated machine guns introduced for bomber defense during the First World War, and so made the bomber and more formidable opponent to attacking fighters.