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.

[1] A conventional biplane, the Sólyom was powered by a 870 hp (649 kW) Weiss WM-K-14A radial 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.

[3] From June 1941 they were used to support Hungarian Army units in Ukraine, and then against Soviet partisans.