The Sturmgeschütz III (StuG III) assault gun was Germany's second most-produced armoured fighting vehicle during World War II after the Sd.Kfz. 251 half-track.
It was built on the chassis of the proven Panzer III tank, replacing the turret with an armored, fixed superstructure mounting a more powerful gun. Initially intended as a mobile assault gun for direct-fire support for infantry, the StuG III was continually modified, and much like the later Jagdpanzer, was employed as a tank destroyer.
The Sturmgeschütz originated from German experiences in World War I, when it was discovered that, during the offensives on the Western Front, the infantry lacked the means to effectively engage fortifications. The artillery of the time was heavy and not mobile enough to keep up with the advancing infantry to destroy bunkers, pillboxes, and other minor fortifications with direct fire. Although the problem was well known in the German army, it was General Erich von Manstein who is considered the father of the Sturmartillerie (assault artillery). This is because the initial proposal was from Colonel Erich von Manstein and submitted to General Ludwig Beck in 1935, suggesting that Sturmartillerie units should be used in a direct-fire support role for infantry divisions. On 15 June 1936, Daimler-Benz AG received an order to develop an armoured infantry support vehicle capable of mounting a 75 mm (2.95 in) calibre artillery piece. The gun mount's fixed, fully integrated casemate superstructure was to allow a limited traverse of a minimum of 25° and provide overhead protection for the crew. The height of the vehicle was not to exceed that of the average soldier.
Daimler-Benz AG used the chassis and running gear of its recent Panzer III medium tank as a basis for the new vehicle. Prototype manufacture was passed over to Alkett, which produced five prototypes in 1937 on Panzer III Ausf. B chassis. These prototypes featured a mild steel superstructure and a Krupp short-barrelled, howitzer-like in appearance, 7.5 cm StuK 37 L/24 cannon. Production vehicles with this gun were known as Gepanzerte Selbstfahrlafette für Sturmgeschütz 7.5 cm Kanone Ausführung A bis D (Sd.Kfz.142).
While the StuG was considered self-propelled artillery, it was not clear which land combat arm of the German Army would handle the new weapon. The Panzerwaffe (armoured corps), the natural user of tracked fighting vehicles, had no resources to spare for the formation of StuG units and neither did the infantry. It was agreed that it would best be employed as part of the artillery arm. The StuGs were organized into battalions (later renamed "brigades" for disinformation purposes) and followed their own doctrine. Infantry support using direct-fire was its intended role. Later, there was also a strong emphasis on its use as an anti-tank gun.