In March of 1942, the StuG 40 Ausf. F entered production. The main feature of the new vehicle compared to the StuG III was a 75 mm StuK 40 gun with a 43 caliber long barrel. Thanks to the new gun, the vehicle became a tank destroyer. Formally, it was still called an "assault gun", but the new SPG was developed specifically to fight tanks. However, the German military understood that even though the new gun had excellent characteristics, wartime conditions of rapidly growing calibers and armour meant that it will soon need replacing.
The wait for a new gun was not long. In the winter of 1942, Rheinmetall-Borsig developed the 75 mm KwK L/70 gun, initially meant for the VK 45.01(H) heavy tank. The idea to equip these tanks with these weapons in a turret also developed by Rheinmetall-Borsig did not move past a full sized model. The gun got lucky: it made its way into the VK 30.02 (MAN) project, which, with some changes, was approved for production as the Pz.Kpfw. Panther. The choice of gun proved to be a correct one: the KwK 42 did not only become the Panther's main armament, but outlived the tank. A shortened version of the KwK 42 was used by the French on AMX 13 tanks and Panhard armoured cars. It was also used on Israeli M50s.
Designing a gun for tank destroyers based on the KwK 42 was logical. Interestingly enough, the first chassis for the new tank destroyer was not a medium tank. On April 12th, 1942, development of the Sturmgeschuetz auf Leopard (L/70) began, based on the Leopard reconnaissance tank. An assault gun on the same chassis with a 105 mm leFH 18 howitzer in a lightly armoured casemate was also planned. While that project at least reached the stage of a full sized model, the Sturmgeschuetz auf Leopard (L/70) was not as lucky. On January 3rd, 1943, Hitler personally cancelled work on the Leopard project, and even the draft of the tank destroyer was not finished.
In late January of 1943, the 7.5 cm Pak 42 L/70 mit Kugelblende auf VK 9.03 project was proposed. It was also cancelled in favour of the VK 13.03 project (the Luchs). The tank destroyer on the VK 9.03 chassis was rather awkward. Looking at it, one inevitably asks the question: how is it possible to drive with such a huge "fishing rod" that's longer than the entire hull?