The World War I German military were especially interested in the development of these superweapons due to the need for the Schlieffen plan to march past a line of Belgian fortifications constructed specifically to stop such an invasion route. During the opening phases of the war, the Germans employed a 420 mm Krupp howitzer (the Big Bertha) and two 305 mm Skoda mortars to reduce the famous fortresses of Liège and Namur. Their low overland mobility made them arrive later than the infantry at Liège, so several infantry assaults were made with heavy loss of life and generally little success. The guns arrived a few days later and reduced the fortresses at Liège one-by-one over a short period of a few days.
Larger artillery after this opening period was generally limited to railway guns, which had much greater mobility, or naval monitors (two of the British Lord Clive class monitors were fitted with an 18 inch (457 mm) gun, and HMS General Wolfe fired 33 km at a railway bridge in Belgium). All of the major powers involved employed such weapons in limited numbers, typically between 280 and 305 mm (11 to 12 inches) although some larger weapons were also used. The largest of the railway guns deployed in World War I was the Paris Gun, which was used to bombard Paris from a distance of over 130 kilometres (80 miles).
Development continued during the inter-war era, although at a more limited pace as aircraft were expected to take over in the long-range bombardment role. Nevertheless the Germans built a handful of powerful Krupp K5's and the great 800 mm Schwerer Gustav (and Dora). The latter had been designed specifically to defeat the Maginot Line, firing a 7 ton shell to a range of 37 km. Although their original role proved unnecessary, Gustav was used successfully to demolish several heavy fortifications, most notably those at Sevastopol. Dora was readied for combat at Stalingrad, but was withdrawn before it could be used. Gustav and Dora were the largest artillery pieces (by caliber) ever used in combat. Development may have ended there but for the ever-increasing Allied air power, which limited Hitler's options in terms of re-opening bombing attacks on London. This led to the development of the V-3 "London Gun" or "Hochdruckpumpe", fired from a site in the Pas de Calais, about 95 miles (153 km) away. Two attempts to build underground bunkers for the huge weapons were thwarted by massive Royal Air Force bombing raids, which made further attempts futile. Two smaller prototype version of the gun were used during the Battle of the Bulge.
Canadian-born engineer Gerald Bull later became interested in the possibility of using superguns in place of rockets to insert payloads into orbit. He started Project HARP to investigate this concept. HARP was later cancelled, and Bull turned to military designs, eventually developing the GC-45 howitzer, now the basis of every modern artillery piece. Some years later, Bull interested Saddam Hussein in funding Project Babylon. The objective of this project is not certain, but it is thought to have been intended to develop a gun capable of firing an object into orbit, from where it could then drop onto any place on the Earth. Gerald Bull was assassinated, allegedly by Israeli Mossad, terminating development, and the parts were confiscated by British customs after the Iraq War.