(the "B-17F-xx-DL" designated blocks), and for all versions of the B-17G Flying Fortress a twin-gun remotely operated "chin" turret, designed by Bendix and first used on the experimental YB-40 "gunship" version of the Fortress, was added to give more forward defence. Specifically designed to be compact and not obstruct the bombardier, this was operated by a swing-away diagonal column possessing a yoke to traverse the turret, and aimed by a reflector sight mounted in the windscreen. The intended replacement for the German Bf 110 heavy fighter, the Messerschmitt Me 210, possessed twin half-teardrop-shaped, remotely operated Ferngerichtete Drehringseitenlafette FDSL 131/1B turrets, one on each side "flank" of the rear fuselage to defend the rear of the aircraft, controlled from the rear area of the cockpit. By 1942, the German He 177A Greif heavy bomber would feature a Fernbedienbare Drehlafette FDL 131Z remotely operated forward dorsal turret, armed with twin 13mm MG 131 machine guns on the top of the fuselage, which was operated from a hemispherical, clear rotating "astrodome" just behind the cockpit glazing and offset to starboard atop the fuselage—a second, manned powered Hydraulische Drehlafette HDL 131 dorsal turret, further aft on the fuselage with a single MG 131 was also used on most examples. The US B-29 Superfortress had four remote controlled turrets, comprising two dorsal and two ventral turrets. These were controlled from a trio of hemispherically glazed gunner-manned "astrodome" sighting stations operated from the pressurised sections in the nose and middle of the aircraft, each housing an altazimuth mounted pivoting gunsight to aim one or more of the unmanned remote turrets as needed, in addition to a B-17 style flexible manned tail gunner's station. The defensive turret on bombers fell from favour with the realization that bombers could not attempt heavily defended targets without escort regardless of their defensive armament unless very high loss rates were acceptable, and the performance penalty from the weight and drag of turrets reduced speed, range and payload and increased the number of crew required. The already mentioned British de Havilland Mosquito light bomber was designed to operate without any defensive armament and used its speed to avoid engagement with fighters, much as the minimally armed German Schnellbomber aircraft concepts had been meant to do early in World War II. A small number of aircraft continued to use turrets however—in particular maritime patrol aircraft such as the Avro Shackleton used one as