Production started in 1935 and about 1650 were made. xxxxx Mk.III SD (Special Duties) for clandestine operations carrying agents or VIPs to and from enemy territory. Extra fuel capacity with a 150 Imperial gallon (682 litre) auxiliary fuel tank and access ladder to rear cockpit on left side, armoured floor, improved radio equipment and provision for two 'passengers'. It had a 870 hp (649 kW) poppet-valve 9-cylinder air-cooled radial engine rated at 2,650 rpm at 4,500 ft (1370 m). Maximum speed 212 mph (341 km/h) at 5,000 ft (1525 m); service ceiling 21,500 ft (6555 m); stalling speed 56 mph (90.1 km/h); climb to 10,000 ft (3048 m) in 8 minutes. Wingspan 50 ft 0 in (15.24 m); length 30 ft 6 in (9.30 m); height 14 ft 6 in (4.42 m); wing area 260.0 sq ft (14.15 sq m).