variant of the SPAD S.A two-seat fighter was probably the first aircraft to be fitted with a remotely-controlled gun, which was located in a nose nacelle. As aircraft flew higher and faster, the need for protection from the elements led to the enclosure or shielding of the gun positions, as in the "lobsterback" rear seat of the Hawker Demon biplane fighter. The first British operational bomber to carry an enclosed, power-operated turret was the British Boulton & Paul Overstrand twin-engined biplane, which first flew in 1933. The Overstrand was similar to its First World War predecessors in that it had open cockpits and hand-operated defensive machine guns. However, unlike its predecessors, the Sidestrand could fly at 140 mph (225 km/h) making operating the exposed gun positions difficult, particularly in the aircraft's nose. To overcome this problem, the Overstrand was fitted with an enclosed and powered nose turret, mounting a single Lewis gun. As such the Overstrand was the first British aircraft to have a power-operated turret. Rotation was handled by pneumatic motors while elevation and depression of the gun used hydraulic rams. The pilot's cockpit was also enclosed but the dorsal (upper) and ventral (belly) gun positions remained open, though shielded. The Martin B-10 all-metal monocoque monoplane bomber introduced turret-mounted defensive armament within the United States Army Air Corps, almost simultaneously with the RAF's Overstrand biplane bomber design. The Martin XB-10 prototype aircraft first featured the nose turret in June 1932—roughly a year before the less advanced Overstrand airframe design—and was first produced as the YB-10 service test version by November 1933. The production B-10B version started service with the USAAC in July 1935. In time the number of turrets carried and the number of guns mounted increased. RAF heavy bombers of World War II such as the Handley Page Halifax (until its Mk II Series I (Special) version omitted the nose turret), Short Stirling and Avro Lancaster typically had three powered turrets: rear, mid-upper and nose. (Early in the war, some British heavy bombers also featured a retractable, remotely-operated ventral (or mid-under) turret). The rear turret mounted the heaviest armament: four 0.303 inch Browning machine guns or, late in the war, two AN/M2 light-barrel versions of the US Browning M2 machine gun as in the Rose-Rice turret. The tail gunner or "Tail End Charlie" position was generally accepted to be the most dangerous assignment. During the World War II