In the second half of the 1930’s, any sound warplane that was generally available could be sure of attracting widespread interest.

The Fokker D.21. came from a company with a great reputation all over the world, and though it was designed - by Ir. E. Schatzki in 1935 – purely to meet the requirements of the Netherlands East Indies Army Air Service, it became the leading fighter of three major European nations and was planned as a standard type by a fourth.

This was as well for Fokker, because the plans of the original customer were changed and a contract was never signed.

Yet the little fighter was all one could expect: neat, tough and highly maneuverable, with good performance and heavy armament. It marked the transition between the fabric-covered biplane and the stressed-skin monoplane.

The wing was wood, with bakelite/ ply skin. The fuselage was welded steel tube, with detachable metal panels back to the cockpit and fabric on the rear fuselage and tail. The landing gear was fixed.

There were many Fokker projects for developed D.21.’s with retractable landing gear and other engines, but the production aircraft was generally similar to the prototype.

In the seventh prototype (No. 217) test pilot H. Leegstra set  a Dutch height record at 37.250 ft / 11.400 m.

Meanwhile, production of a modified version was getting under way for Finland, which bought seven aircraft with a manufacturing license.

Denmark followed with an order for three and a manufacturing

license and a fourth to adopt the D.21. was Republican Spain.

The latter set up a new plant and was about to start accepting deliveries, when the area was overrun by nationalist forces.

The VL (Finnish State factory) delivered 38 in 1939- 1939 and all of them participated very successfully in air battles against the Soviet forces from the start of the Soviet invasion, but all the Finnish-built Mercuries were needed for Blenheims.

The Finnish D.21. was redesigned to take the heavier but less powerful Twin Wasp Junior, 55 of this type being built, one of these having a retractable landing gear.

The Danish Royal Army Aircraft Factory gradually delivered ten aircraft with low-rated Mercury engines and two cannons, eight being taken over during the German Invasion in March 1940.

Finally, on 10 May 1940, the 29 combat ready aircraft in Holland fought round the clock until their ammunition ran out the third day.
The Dutch gave up fighting on 14 May 1940.

The 241 is shot and set on fire during the landing. The 219 is aborting the landing and the 218 is taking off again. There is a bitter fight between seven Dutch aircraft and these nine German Bf-109’s.

The 219 is destroying the engine of one Bf-109, forcing it to make a crash landing. The 234 and the 242 both gun down a Bf-109. The 221 is damaging one of the Bf-109’s so badly, that it is going down in the “Wieringermeer” lake.

The 218 and the 234 arechasing two other Germans and force them to disengage. One of these two Bf-109’s crash lands on one of the German North Sea Islands. The remaining nine D-XXI aircraft all land at De Kooy, all with battle damage, five of them have serious battle damage.

The 214 and the 233 are destroyed shortly after that due to shelling by Bf-109 and Me-110 aircraft.

After four D-XXI’s are repaired, there is a bombardment with firebombs by Me-110 aircraft. The 234 is lost in this bombardment while six others are seriously damaged. The remainder of 1e JAVA is being joined together with what is left of the 2nd JAVA (SCHIPHOL) at the auxiliary airfieldBUIKSLOOT nearby Tuindorp-Oostzaan.  

2nd JAVA is posted at SCHIPHOL and has nine operational D-XXI aircraft. They take off during a bombardment of the airfield at 03.58 h.

1e and 2nd JAVA are now operating together and they have eight Fokker D-XXI aircraft available.

- On 11 May three D-XXI’s will escort two Fokker C-V biplanes to “Ede / Arnhem”, but there is a misunderstanding during this mission and the D-XXI’s return to Buiksloot. Three other D-XXI’s, being repaired at De Kooy airfield, land there.

Three other D-XXI aircraft escort Fokker T-V bombers on a mission to Rotterdam to destroy the “Maasbrug” (bridge over the river “Maas). All aircraft return intact.

Again three D-XXI’s are escorting two C-V biplanes to “Ede / Arnhem” and these planes too return intact.

The first effort on bombing the Maasbrug failed, so a second mission is planned. Again the T-V bombers are being escorted by three D-XXI’s. They meet twelve Messerschmidt Me-110 aircraft. The 242 escapes and lands at Buiksloot. The pilot of the 225 decides to use his parachute.
He has already released his canopy, this canopy hits the propeller of a Me-110. He flies through a cloud and with his machine guns he hits a second Me-110. After that he is shot down himself. His aircraft ends up in the surroundings of “Leiden” city.

The 213 lures away six Me-110’s from the T-V bombers. One of the enemy planes is shot down over “Moerkapelle”, probably a second one. However, the pilot of the 213 is seriously injured and lands on the road between Den Haag (the Hague) and Utrecht, near “Zevenhuizen”.

Nine Fokker D-XXI aircraft are left at Buiksloot airfield. In the afternoon, two of the planes are going on a recce mission over western Amsterdam. It is believed that paratroopers have landed there but they are not spotted.

For a long time they are being shot at, again by friendly forces.

The 236 is being hit and crash lands near “Vleuten”. The remaining aircraft land at Schiphol, as ordered.

As a result of the many repairs, the material and equipment is wearing out very fast, the power of the Germans is too much. A list ist being put together with names of possible “Engelandvaarders”, people that evacuate to Great Britain.

However, five D-XXI’s still go on a patrol mission. They encounter no enemy aircraft and return to base safely. The aircraft are being prepared for a next mission but there is almost no ammunition left.

The remaining nine D-XXI see no action any more. At 19.00 h that 14th May, the Dutch surrender and the submission is being signed. Crossing to England is strictly forbidden, as being against submission rules.

At YPENBURG Airpark, nine D-XXI aircraft of the 1e Afdeling Jachtgroep Veldleger (1st Division Fighter Group Field Army) are detached.

When German aircraft are approaching at 04.00 h, eight D-XXI aircraft that are battle-ready, take off.
They fly in two formations of three and one of two aircraft.

The formations however, are being driven apart by the Germans and than the D-XXI pilots engage the German aircraft by themselves.

The 222 is engaging but has problems with jamming guns. The aircraft heads west but is attacked by German planes and crash lands near “Monster”.

On Ypenburg airfield, the backward unarmed D-XXI is a prey for German machine guns.

The 246 has four jamming guns. The pilot lands his plane and takes off again in the 215. Later on he lands the undamaged airplane on the beach at “Kijkduin”, where the aircraft is destroyed by German fighters.

The 212 is taking part in the battle until the aircraft runs out of fuel and then heads for Schiphol. Near “De Kaag” this D-XXI shoots down a Dornier D0-17, but during this attack his engine receives damage.

The D-XXI glides down to Schiphol, where it lands. There the 212 is attached to 2nd JAVA.

The aircraft is being damaged during a German attack.

At the end of the fighting, the Militaire Luchtvaartafdeling (Military Aviation) had lost 20 officers, 25 Non Commissioned Officers and 30 corporals and airmen.

On 18 May 1940 the “Wapen der Militaire Luchtvaart” (the Military Aviation) is awarded the “Militaire Willemsorde 4e klasse” (Military Williams medal 4th class) by the supreme commander of the Army and the Navy.