It was initially designed as an infantry-support medium tank (Begleitwagen, mittlerer Panzer), to work in conjunction with the Panzer III which was intended to engage enemy tanks. Later in the war it was up-gunned and up-armored and took over the tank-fighting role while Panzer IIIs were either put into infantry support duties or converted into other vehicles. The Panzer IV was the most common German tank of World War II, and was used as the base for many other fighting vehicles, such as tank destroyers and self-propelled anti-aircraft guns. The Panzer IV was the workhorse of the German tank corps, being produced and used in all theatres of combat throughout the war. The design was upgraded repeatedly to deal with the increasing threats from enemy forces. The Panzer IV has the distinction of being the only German tank to remain in continuous production throughout all of World War II, with over 8,500 produced from 1937 to 1945.