The Panzer IV series was developed in the early 1930s as the next step in the evolution of German armor. Where the Panzer I and II were light tanks that spearheaded the early Blitzkrieg, the Panzer III series was envisioned as the tank fighter while the Panzer IV was to be the infantry support tank, initially armed with the 75mm L/24 main gun. When the push into the Soviet Union began, the early Panzers were soon to be outgunned by Soviet armor. While the Panzer III and Panzer IV were both up-armored, the smaller turret ring of the Panzer III limited the firepower upgrades that could be applied to that series. The Panzer IV featured a larger turret ring that could accommodate the continued growth of turret and main gun. While the Panther was intended to replace the Panzer IV, the latter remained in production and in service through the end of the war.