In 1936 a few Fokker D.XXIs were used by the Spanish Republic. Although the order by the ML-KNIL was cancelled, the Luchtvaartafdeling (Dutch Army Air Force before World War II) placed an order of 36 aircraft, which were all delivered in time to participate in the war against the Germans in May 1940. The Fokker D.XXI, although much slower and more lightly armed than the Bf109, performed surprisingly well in dogfights, due to its maneuverability. It was also one of the few aircraft that could follow a Stuka bomber into its dive. Nonetheless, the numerical inferiority of the Luchtvaartafdeeling compared to the Luftwaffe resulted in the destruction of most Dutch Fokker D.XXI fighters during the campaign. Some were captured during and after 15 May, but their fates, apart from their capture, are unknown.
The Fokker D.XXI performed better and for much longer in the Finnish Air Force, which had acquired a number of licence-built fighters prior to the start of the Winter War. Against the aircraft of the soviet Air Force, the Fokker was more evenly matched, and its rugged design with a radial engine and fixed undercarriage made it very suitable for Finnish conditions. Later in the war, as newer models of Soviet fighters appeared, the Fokker D.XXI was underpowered and too lightly armed (with only four 7.92 mm/.312 in machine guns) to compete. Plans to arm the Fokkers with 20 mm cannons were dropped and only one fighter was armed as such (two 20 mm cannons and two 7.92 mm/.312 in machine guns). Another fighter was equipped with retractable landing gear, but due to less than anticipated performance improvement, wasn't continued in the series. During the Continuation War (1941–44) the Finnish State Aircraft Factory (Valtion Lentokonetehdas, VL) also built some 50 D.XXIs with the Swedish-built Pratt & Whitney R-1535 Twin Wasp Junior as the Bristol Mercury was in short supply. These can be identified by their longer cockpit glazing, smooth cowl, and large ventral air intake under the cowl. The fixed undercarriage lent itself to both unimproved runways and conversion to skis for winter use, both of which were advantages in the Finnish theater.
Several Finnish Air Force pilots became fighter aces on Fokker D.XXI, top scoring Fokker ace being Jorma Sarvanto with 12 5/6 victories. Many other aces scored at least one victory on Fokker. The highest scoring airframe is FK-110, with 10 victories, which survived the war, and is on display at Central Finland Aviation Museum.
Vier uitvoeringen mogelijk;
- Fokker D,.XXI, No.234/constuction number 5507, of the 1st JaVA (1st Fighter Squadron), ML (Militaire Luchtvaart, the Royal Netherlands Air Force), April-May 1940. TltBram "Bob" van der Stok, whie at the controls of this aeroplane, shot down a Bf109E and damaged another one. During the landing at the De Kooij airfield the aircraft No.234 was attacked by another Bf109E, itwas shot to pieces and burnt down. B.van der Stock luckily escaped from de cockpit.
- Fokker D.XXI, No.221/constuction number 5494, of the 1st JaVA (1st Fighter Squadron), ML (Militaire Luchtvaart, the Rpyal Netherlands Air Force), April-May 1940. On 10 May 1940 thisaeroplane N o.221, was piloted by Tlt.Herman Doppenberg. He alsoshot dwon a Bf109E during that morning aerial combat above the De Kooij airfield. The aircraft was destroyed during the attack on the Schiphol airfield on 14 May.
- Fokker D.XXI, No.231/construction nimber 5504, of the ML vliegschool Texel (the Royal Netherlands Air Force Pilot School), Texel air base, April-May 1940. This aircraft was captured by the German Luftwaffe as is shown here.
- Fokker D.XXI, No231/contruction number 5504, capured by the Luftwaffe . After its capture the aircraft No.231 was initially tested at Rechlin, wearing the German markings but retaining the original number and the rudder in the ML colours, Later, the Dutch marks were completely painted over and the Machiene received the proper Luftwaffe markings. However, the swastika on the vertical tail was not correctly a pplied according to the regulations, but it corresponded to the position of the aircraft when parked on the ground. In the autumn of 1940 this aeroplane was used by the Fluglehrerschule at Brandenburg-Briest. 1940 it crashed there and destroyed, with its German injured.