The WM-21 was designed to replace the WM-16, which was based on the yet older Fokker C.V, and as such was considered unsuitable for operational service.[citation needed] The WM-21's structure was strengthened, and the aircraft received a new, more efficient wing set. A tailskid was fitted to allow for shorter landing runs on grass airfields.[1] A conventional biplane, the Sólyom was powered by a 870 hp (649 kW) Weiss WM-K-14Aradial engine, and had an open cockpit.[1] A total of 128 aircraft were built by three different factories: 25 by Manfréd Weiss, 43 by MÁVAG, and 60 by MWG.[2]
During the invasion of Yugoslavia, none of the WM-21s were lost in combat, but one was lost in an accident.[3] From June 1941 they were used to support Hungarian Army units in Ukraine, and then against Soviet partisans.[1] They lost another WM-21 on June 29, when the war against the Soviet Union was intensifying.[4] Around 80 aircraft were also transferred to duties as trainers, as they were removed from operational use, until 1945.[1]