Entering operational use in the early years of the Second World War, it provided yeoman service for both the Luchtvaartafdeling (Dutch Army Aviation Group) and the Finnish Air Force.

Additionally, a few examples were constructed by the El Carmolí factory before it fell into rebel hands during the Spanish Civil War.

On 14 November 1934, design proposals for a new fighter aircraft were submitted by Fokker to the Luchtvaartafdeling (Dutch Army Aviation Group).

[2] In early 1935, the Luchtvaartafdeling signed a contract for a single prototype of the proposed fighter to be constructed for an evaluation to be performed by the Royal Netherlands East Indies Army.

[2] This prototype, designated FD-322, which was powered by a single Bristol Mercury VI-S radial engine which drove a three-blade, two-pitch propeller, performed its maiden flight at Welschap Airfield, Eindhoven, on 27 March 1936.

Kamphuis, the prospects for series production of the new fighter looked doubtful shortly after the first flight was performed due to a high level change in Dutch defence policy; Minister for Foreign Affairs Hendrik Colijn informed the Ministry of War that, in response to the changing international situation, a higher priority would be placed on building up a substantial bomber capability over new fighter aircraft.

[4] It was decided that the D.XXI and F.K.58 should participate in a series of comparative tests against one another, leading to the D.XXI prototype being dispatched to Soesterberg Air Base, Utrecht, in November 1936.

[4] During 1937, the Dutch government gave funding and its approval for a limited expansion of the Army Aviation Group, which resulted in an order being placed for 36 Fokker D.XXI fighters, to be powered by the 830 h.p.

The two fuselage-mounted machine guns on these D.XXIs were relocated in the wings and the vertical tail surface was increased in size to offset the new engine.

[2] The aircraft was outfitted with a fixed spatted undercarriage with cantilever legs; braking was provided by independently-operated pedals using compressed air.

[2] The cockpit of the D.XXI was fully enclosed by a plexiglas hood featuring large sliding sections, and was entirely jettisonable in an emergency situation to enable pilots to bail out.

[2] Upon its entry to service in 1938, the D.XXI represented a significant leap forward for the Dutch Army Aviation Group, whose fighter force had until that time consisted of aging biplanes with open cockpits.

[9] The Fokker was evenly matched against the aircraft of the Soviet Air Force, and its rugged design with a radial engine and fixed undercarriage made it well suited for Finnish conditions.

[citation needed] Another fighter was equipped with retractable landing gear, but due to less than anticipated performance improvement was not continued in the series.

[10] The conflict between Finland and the Soviet Union was resumed in the Continuation War (1941–1944), the D.XXI was again a key element of the Finnish Air Force.

The highest scoring airframe was FR-110, achieving 10 victories; this aircraft survived the war and is on display at the Central Finland Aviation Museum.

[8] That same day, a flight of D.XXIs intercepted and shot down 37 out of 55 inbound Junkers Ju 52 transports which had crossed the border during the early morning.

[11] Due to many aircraft becoming unserviceable as a result of battle damage after the first day, it was decided to regroup at Buiksloot, north of Amsterdam, on 11 May.

For the following four days, missions out of Buiksloot were flown by D.XXIs flying in both solo and small formations to escort friendly units as well as in the search-and-destroy role.