The Stirling was armed with nose and tail turrets (the latter was notable for the wide angles of fire) along with a single retractable ventral ("dustbin") turret located just behind the bomb-bay. This proved almost useless due to cramped conditions, with the added distraction that the turret tended to drop and hit the ground when taxiing over bumps. The retractable turret was removed almost from the start and temporarily replaced by beam hatches mounting pairs of machine guns, until a twin-gun dorsal turret could be provided. This turret designated the FN.7A also had problems; it had a metal back fitted with an escape hatch which turned out to be almost impossible to use. The Stirling Mk.III, introduced in , was similar to the Mk.I with the exception of the improved 1,635 hp (1,200 kW) Hercules VI or XVI engines, which improved maximum speed from 255 to 270 mph (410 to 435 km/h). The Mk.III used a fully glazed dorsal turret (the same FN.50 as in Lancaster) that had more room and an improved view. Early Mk.III Stirlings were fitted with a 12.7 mm Browning machine gun in the rear escape hatch (behind a perspex shield) to ward off German night fighters using the Schräge Musik system. Later Stirlings were fitted with an improved, low-drag remotely-controlled FN.64 ventral turret or a H2S radar. Mk.III Stirlings also were fitted with electronic countermeasure systems such as ventral antennas for the Mandrel jamming system as well as a ventral "window" chaff chute to jam Freya and Würzburg radars. Mk.III Stirlings were also fitted with a ventral antenna for the Blind Approach Beacon System which was a blind-landing aid and a Monica rear warning radar in the tail turret.