The Panzer IV was the brainchild of the German general and innovative armored warfare theorist Heinz Guderian. In concept, it was intended to be a support tank for use against enemy anti-tank guns and fortifications. Ideally, each tank battalion in a panzer division was to have three medium companies of Panzer IIIs and one heavy company of Panzer IVs. On 11 January 1934, the German army wrote the specifications for a "medium tractor", and issued them to a number of defense companies. To support the Panzer III, which would be armed with a 37-millimetre (1.46 in) anti-tank gun, the new vehicle would have a short-barreled, howitzer-like 75-millimetre (2.95 in) as its main gun, and was allotted a weight limit of 24 tonnes (26.46 short tons). Development was carried out under the name Begleitwagen ("accompanying vehicle"), or BW, to disguise its actual purpose, given that Germany was still theoretically bound by the Treaty of Versailles ban on tanks. MAN, Krupp, and Rheinmetall-Borsig each developed prototypes, with Krupp's being selected for further development.
The next version, the Ausf. H, began production in June 1943 and received the designation Sd. Kfz. 161/2. The integrity of the glacis armor was improved by manufacturing it as a single 80-millimetre (3.15 in) plate. A reinforced final drive with higher gear ratios was introduced. To prevent adhesion of magnetic anti-tank mines, which the Germans feared would be used in large numbers by the Allies, Zimmerit paste was added to all the vertical surfaces of the tank's armor. The turret roof was reinforced from 10-millimetre (0.39 in) to 16-millimetre (0.63 in) and 25-millimetre (0.98 in) segments. The vehicle's side and turret were further protected by the addition of 5-millimetre (0.20 in) hull skirts and 8-millimetre (0.31 in) turret skirts. This resulted in the elimination of the vision ports located on the hull side, as the skirts obstructed their view. During the Ausf. H's production run, its rubber-tired return rollers were replaced with cast steel, a lighter cast front sprocket and rear idler wheel gradually replaced the previous components, the hull was fitted with triangular supports for the easily damaged side skirts, the Nebelwurfgeraet was discontinued, and a mount in the turret roof, designed for the Nahverteidigungswaffe, was plugged by a circular armored plate due to initial production shortages of this weapon. Despite addressing the mobility problems introduced by the previous model, the final production version of the Panzer IV—the Ausf. J—was considered a retrograde from the Ausf. H. Born of necessity, to replace heavy losses, it was greatly simplified to speed production.
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