This two-seat biplane had tandem open cockpits, with dual controls; it was armed with two 7.7-mm (0.303-in) forward-firing machine-guns in the upper wing, a third movable 7.7-mm (0.303-in) gun in the rear cockpit, and provision for under-fuselage bomb racks. A vertical camera was mounted in the floor of the rear cockpit. Usual powerplant was a 350-hp Armstrong Siddeley Cheetah IX radial engine, but some F.K.51s had 270-hp Cheetah V, 450-hp Pratt & Whitney Wasp, or (those for Spain) 400-hp Armstrong Siddeley Jaguar engines; the F.K.51 bis (for Spain) had a 450-hp Wright Whirlwind.
The LVA (Dutch army air service) ordered 53 in 1937, later increasing its order. In all, some 130-140 F.K.51s were built, of which 28 F.K.51/51bis were supplied to Spain during the civil war. When Holland was invaded in May 1940, the 2nd Air Regiment possessed 16 observation F.K.51s. Others were used by the Dutch East Indies army air service for reconnaissance and coastal/maritime patrol during 1941-42.
The FK.51 basic trainer first flew in May 1935. The type was ordered for the Dutch and Netherlands East Indian air forces. With a more powerful engine the type offered a combination of advanced training and limited combat capabilities, and during 1936 was ordered to the extent of 28 aircraft by the Republicans. These were delivered as 11 FK.51 aircraft with the 298-kW (400-hp) Armstrong Siddeley Jaguar VA radial and 17 FK.51bis aircraft with the 336-kW (450-hp) Wright R-975-E radial: some were used as night-flying trainers, but most served as night-fighters and reconnaissance aircraft with two fixed and one trainable machine-guns.
Span: 9m (29ft 6.25 in).
Length: 7.85m (25 ft 9in).
Engine: 1 x Armstrong Siddeley Jaguar VA, 298kW (400 hp).
Armament: provision for 3 x 7.7-mm (0.303-in) mg.
Max T/O weight: 1450kg (3,197 lb).
Max speed: 155 mph at 7,545ft.
Operational range: 513 miles.
Engine: Wright R-975-E, 336-kW (450-hp).