The main design works on the T-21 began as early as September 1936. The first prototype was finished in May 1937 and first variant from May 1937 was the original Š-IIc. The engine development was delayed and the prototype was fitted (in order to save time) with a 190 hp 13-liter V6, originally intended for the Š-III breakthrough tank prototype. In September 1937, the V6 engine was removed and the original Škoda engine intended for it was installed. It still failed its tests which were stopped in November, marking the end of the 1st development stage of T-21. By that time the Ministry of Defence committee was looking for a suitable Czechoslovak medium tank for the army but the Š-IIc did not to make the June 1938 army tests deadline and as a result this was the end of the T-21 as a potential Czechoslovak army medium tank. In any case, after the German occupation of Czechoslovakia that was no longer possible, not without German consent at least. During the early months of occupation German delegations visited the Škoda factory and tests were performed with the Š-IIc prototype which, by 22.5.1939, had been renamed, to fit the German nomenclature principles to Škoda T-21 (T = tank, 2 = medium, 1 = 1st variant). The Germans initially wanted to test it at Kummersdorf, but did not show too much interest in the design and instead the Germans decided to produce an improved version, which was named T-22.