In Service with LVA and NEIAF.

NEIAF

After the first flights of the factory the D.XXI was tested by the ML and by the RSL (National Research Centre for Aviation). The two-bladed propeller is replaced by a three-bladed propeller Ratier.

Between late November 1936 and mid-January 1937 the D.XX.
It was also tested by the LVA at Soesterberg.
Probably in January 1937, the Fokker D.XXI was shipped to the Dutch East Indies. There it made its first flight April 16, 1937 from Andir.

The NEIAF pilots were not very enthusiastic about the D.XXI and also many problems occurred.
It is uncertain what happened with the Fokker D.XXI; some sources state that it was scrapped in 1941; other sources state that it was destroyed in February 1942 during a bombardment of Kalidjati.

LVA

Partly due to the priority for training aircraft and so called "cruisers" and also by the relatively low gain in speed (maximum speed 411 km/hr ) compared with the Fokker D.XVII ( maximum speed 370 km/hr) the LVA showed little interest in the D.XXI .

The LVB (equipment service of the LVA) noticed slow delivery of orders for foreign aircraft, so the LVB decided to consider the D.XXI after all, though it had to undergo a number of changes so the LVA version corresponded much with the Finnish version of the D.XXI.

The performance was not as required for a fighter, but considered good enough to protect scouts and for ground attack tasks, so in December 1936 Fokker received an order for 36 examples.

The LVA thought to be able to buy more modern, faster fighter aircraft. The Hawker Hurricane was one of the candidates, but in 1939 a series Koolhoven Fk.58 's was ordered, but these were never delivered.
As the engine the Bristol Mercury VIII  rated 830 hp was selected, while the armament consisted of four fixed machine guns in the wings.
The first example, wit serial 212, made ​​its first flight on May 26, 1938 , and was , after testing, handed over officially on July 22, 1938.
The D.XXI was equipped with outdated line visors, but there were more modern Revi reflex sights ordered, which, were never delivered due to the invasion.
Autumn 1939 was radio equipment became available, but due to the slow delivery and other problems not all D.XXIs equipped with radio in May 1940.