It was developed in the late 1930s by Krupp in Rügenwalde as siege artillery for the explicit purpose of destroying the main forts of the French Maginot Line, the strongest fortifications in existence at the time.

[3] Gustav was later deployed in the Soviet Union during the Battle of Sevastopol, part of Operation Barbarossa, where, among other things, it destroyed a munitions depot located roughly 30 m (98 ft) below sea level.

Gustav was destroyed by the Germans near the end of the war in 1945 to avoid capture by the Soviet Red Army.

[5] Schwerer Gustav was the largest-calibre rifled weapon ever used in combat, and in terms of weight, the heaviest mobile artillery piece ever built.

[6] It was surpassed in calibre only by the British Mallet's Mortar and the American Little David bomb-testing mortar—both at 36 inches (91.5 cm)—but was the only one of the three to go into action.

In 1934, the German Army High Command (Oberkommando des Heeres (OKH)) commissioned Krupp of Essen to design a gun to destroy the forts of the French Maginot Line that were nearing completion.

[7] Krupp engineer Erich Müller calculated that the task would require a weapon with a calibre of around 80 centimetres (31 in), firing a projectile weighing seven tonnes (15,000 lb) from a barrel 30 metres (98 ft) long.

Technical complications in the forging of such massive pieces of steel made it apparent that the original completion date of early 1940 could not be met.

In November 1941, the barrel was taken to Rügenwalde [de], now Darłowo, Poland, where eight further firing tests were carried out using the 7,100 kilogram armour-piercing (AP) shell out to a range of 37,210 metres (23.12 miles).

In combat, the gun was mounted on a specially designed chassis, supported by eight bogies on two parallel railway tracks.

An extremely-long-range rocket projectile was also planned with a range of 150 kilometres (93 mi), that would require the barrel being extended to 84 metres (276 ft).

In February 1942, Heavy Artillery Unit (E) 672 reorganised and went on the march, and Schwerer Gustav began its long ride to Crimea.

[15] The following targets were engaged: By the end of the siege on 4 July the city of Sevastopol lay in ruins, and 30,000 tons of artillery ammunition had been fired.

It was deployed briefly during the Battle of Stalingrad, where the gun arrived at its emplacement 15 kilometres (9.3 mi) west of the city towards the end of August 1942.

The Monster was to be a 1,500 tonne mobile, self-propelled platform for an 80-cm K (E) gun, along with two 15 cm sFH 18 heavy howitzers, and multiple MG 151 autocannons normally used on combat aircraft.