In the same year seven Mercury VIII powered D.XXIs were ordered for the Finnish air force, and a licensed production arrangement was concluded under which the Finnish State Aircraft Factory at Tampere built 93 D.XXIs between 1938 and 1944. Of these, the first 38 Mercury-powered aircraft were completed by 1938, but a further 50 built in 1941 were modified to take the 825 hp (615 kW) Pratt & Whitney Twin Wasp Junior SB4-C/-G engine, 80 of which had been bought in 1940. Finnish D.XXIs had all four guns mounted in the wings, instead of two in the fuselage and two in the wings. A final batch of five Bristol Pegasus-powered D.XXIs appeared in 1944.

Dutch-built aircraft were delivered to Denmark and a further 10 were built in the Royal Army Aircraft Factory, Copenhagen. They had Mercury VI-S engines and one 20 mm Madsen cannon in a fairing beneath each wing. Licence production of the D.XXI in Spain by the Republican government was begun, but the assembly lines were captured by the Nationalists and it is thought that no D.XXIs were completed. Meanwhile, in Holland the first D.XXI for the Nether- lands air force flew on 20 July 1938, and the last of 36 production aircraft was handed over on 8 September 1939.

At the time of Germany's invasion of the Netherlands on 10 May 1940 (totally the Dutch Air Force had 36 Fokker D-XXI aircraft, registrations were 212 to 247 in white numbers on both sides of the fuselage). Unfortunately, when the war started, only 28 28 D.XXIs were operational. The 28 operational Fokker’s are divided in 1e JAVA (De kooy airfield, near Den Helder), 2nd JAVA (Schiphol, near Amsterdam) and the 1e Afdeling Jachtgroep Veldleger (1st Division Fighter Group Field Army) at Ypenburg Airpark, near Rotterdam. At De Kooy are 11 D-XXI’s. They are divided in in three sections of 3 and one section of 2 aircraft. Only the aircraft of the commanders are equipped with radio’s, so there is no communication possible between the pilots in the aircraft. 2nd JAVA is posted at Schipol and has nine operational D-XXI aircraft.

The manoeuvrability of the Dutch fighters stood them in good stead during the five days before the Netherlands capitulated, although eventually they were overwhelmed by sheer weight of numbers (and limited ammunition supplies) and only eight remained airworthy. Their greatest victory came on 10 May, when D.XXIs destroyed 37 out of a formation of 55 Junkers Ju 52/3ms which crossed the Dutch border in the early morning.

When Finland capitulated to the Russians, on 12 March 1940, 29 D.XXIs were still on strength. It was decided subsequently to build another 50 aircraft, so the Finnish air force had a number available when hostilities with the USSR restarted in June 1941. Some of the D.XXIs began to be replaced by the relatively unsuccessful IVL Myrsky in August that year, although a few lingered on long after the end of the war until finally withdrawn from service in 1948.

Most of the Finnish D.XXIs operated with 'snowshoe' landing gear during the winter. One Twin Wasp-powered aircraft was modified to have retractable landing gear, but its performance was not improved sufficiently to warrant conversion of the other in-service aircraft. A number of pre-war Dutch D.XXIs served as test beds for engines such as the Rolls-Royce Kestrel V and Hispano-Suiza 12Y, and projects were in hand for versions with the Bristol Hercules (Project 150), Rolls-Royce Merlin (Project 151) and Daimler-Benz DB 600H (Project 152), all with retractable landing gear and aerodynamic improvements to the design.

At the present day, there is only one Fokker D-XXI present in the Netherlands, this aircraft is displayed in the Militaire Luchtvaart Museum (Military Aviation Museum) at Soesterberg Airbase. It is a former Finnish aircraft, painted in Dutch markings.

Variants

D.XXI-1 - First production aircraft, delivered to Denmark. They were powered by a Mercury VIS radial, armed with two 7.92 mm (0.312 in) guns in the fuselage and two 20 mm Madsen cannon in large underwing gondolas faired into the undersurface of the wing just outboard of the main landing gear units. Length was 23 ft 7.5 in (7.20 m). Empty weight was 2,701 lbs (1225 kg) with a maximum take-off weight of 3,858 lbs (1750 kg). Maximum level speed was 245.5 mph (395 km/h) at optimum altitude, cruising speed was 207 mph (333 km/h) at optimum altitude. Range was 559 miles (900 km) and service ceiling was 29,530 ft (9000 m). Number built: 2.

D.XXI-2 - The first D.XXI-2 of 36 was handed over in July 1938 to the Dutch air force, the last in September 1939, one week after Germany invaded Poland. Number built: 53.

D.XXI-3 - Finland received 7 D.XXI-2's, and was licensed to build their own aircraft, resulting in the D.XXI-3. 38 were built, and delivered in 1939, and played an important role in the Winter War against the invading USSR-forces. The D.XXI-3 was equal to the D.XXI-2 in all respects. The powerplants were one PZL or Tampella (Bristol) Mercury VII radial engine made in Poland or Finland respectively. Number built: 38.

D.XXI-4 - The D.XXI-4 was powered with a different engine as supplies of the Finnish-built Mercury radial were earmarked for the VL license-built version of the Bristol Blenheim. Finland had bought 80 Twin Wasp R-1535-SB4C-G Junior engines, rated at 825 hp (615 kW), from the USA in 1940, and production of the D.XXI-4 with this engine totalled 50 aircraft. This variant had a slight degradation of performance and agility. Other major changes from the D.XXI-3 standard were the armament of four 7.7 mm (0.303 in) machine-guns in the wing leading edges, a larger vertical tail surface for continued directional stability despite the larger engine, and the rearward extension of the cockpit glazing to provide the pilot with larger fields of vision.

The details of the D.XXI-4 included a span of 36 ft 1 in (11.00 m), an aspect ratio of 7.47 and an area of 174.375 sq ft (16.20 sq m). Length was 27 ft 9 in (8.46 m), and height was 9 ft 8 in (2.95 m). It had an empty weight of 3,384 lbs (1535 kg), with a maximum take-off weight of 4,817 lbs (2185 kg). Maximum level speed of 270 mph (435 km/h) at 9,020 ft (2750 m), cruising speed of 220.5 mph (355 km/h) at 9,020 ft (2750 m), climb to 9,845 ft (3000 m) in 4 minutes 30 seconds, and a service ceiling of 31,990 ft (9.750 m). Number built: 50.

D.XXI-5 - Similar to the Fokker D.XXI-4, but with a Bristol Pegasus radial, rated at 920 hp (686 kW). Number built: 5. 

Specifications (Fokker D.XXI)

Type: Single Seat Fighter

Design: Ir. E. Schatzki in 1935 (purely to meet the requirements of the Netherlands East Indies Army Air Service)

Manufacturer: N. V. (Nederlandsche Vliegtuitenfabriek) Fokker in Amsterdam. License-built by Valtion Lentokonetehdas Finland, Haerens Flyvertroppernes Vaerkstader Denmark and the Spanish Republic Government Plant

Powerplant: (Dutch) One 830 hp (619 kW) Bristol Mercury VIII 9-cylinder radial engine driving a Ratier propeller. (Danish) One 645 hp (481 kW) Mercury VIS engine driving a Ratier propeller. (Finnish) One 825 hp (615 kW) Pratt & Whitney R-1535-SB4-G or C Twin Wasp Junior 14-cylinder two-row radial engine driving a Hamilton Standard propeller.

Performance: (Mercury VIII) Maximum speed 286 mph (480 km/h) at 14,500 ft (4420 m); (P&W R-1535) Maximum speed 272 mph (439 km/h); service ceiling (Mercury) 36,090 ft (11000 m), (P&W R-1535) 32,000 ft (9750 m).

Fuel: (D.XXI-2) 77 Imperal gallon (92.47 US gal or 350 litres) internal fuel stores plus provision for 2 wing tanks.

Range: (Mercury) 590 miles (950 km); (P&W R-1535) 559 miles (900 km).

Weight: (Mercury ) Empty 3,180 lbs (1422 kg) with a maximum take-off weight of 4,519 lbs (2050 kg); (P&W R-1535) Empty 3,380 lbs (1534 kg) with a maximum take-off weight of 4,820 lbs (2186 kg).

Dimensions: Span 36 ft 1 in (11.00 m); length 26 ft 10 3/4 in (8.20 m); height 9 ft 8 in (2.95 m); wing area 174.38 sq ft (16.20 sq m).

Armament: (Dutch) Four 7.9 mm (0.312 in) FN-Browning M.36 machine-guns, two in the fuselage (500 rounds per gun) and two in the wings (300 rounds per gun). (Danish) Two Madsen 7.9 mm (0.312 in) machine guns in the wings and two Madsen 20 mm cannons in underwing blisters. (Finnish) Four 7.7 mm (0.303 in) Vickers machine guns in outer wings.

Variants: D.XXI, D.XXI-1, D.XXI-2, D.XXI-3, D.XXI-4, D.XXI-5.

Avionics: Goertz optical tube-sight or the reflector sight Revi 3C or D.

History: First flight 27 March 1936; service delivery (Dutch) January 1938; (Finnish production) June 1938, (Danish production) 1939.