The project was initially given low priority, but following the invasion of the USSR in 1941 and the appearance of heavily armoured Soviet tanks such as the T-34 and KV-1, it was given an increased priority. The first pre-production guns were delivered in November 1941. In April 1942, the "Wehrmacht" had 44 guns in service; by 1943, the Pak 40 formed the bulk of German anti-tank artillery.