Uncompleted U-boat projects

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During World War II the German Navy (Kriegsmarine) considered a number of submarine designs for specialized operations or improving U-boat performance. However, many of these designs did not come to fruition.

The Type III U-boat was a 1934 project for a purpose built minelayer based on the Type 1A U-boat. The Type III U-Boat would have been similar to the Type IA, but with a hull lengthened by 7.5 metres, and a total displacement of 970 tons. The Type III U-Boat was planned to carry an armament of 54 to 75 mines (depending on the type carried), two 105mm deck guns, and one 20mm anti-aircraft gun.
The Type IIIA U-boat was a planned minelayer similar to the Type IA U-boat. The Type IIIA U-boat would have had a larger outer hull than the Type IA, and featured a large, watertight cylindrical hangar on the aft deck, which would have carried two small motor torpedo boats. The boat would have carried 48 mines, and used the smaller boats to help lay and recover mines. The project was dropped, as impractical.
The Type IV U-boat was a planned re-supply and repair U-boat, intended to meet other U-boats at sea and re-supply them with torpedoes, fuel, food/water, and spare parts, and also be capable of performing light-repair work.
Little information is available on the Type V U-Boat other than it was a Walter boat project.
The Type VI U-boat was a planned conversion of Type IA U-boats to run both submerged and surfaced from steam propulsion. Since many nation’s navies had not used steam propulsion in ships in 30-years, the project was quickly dropped.
The Type VIIE U-boat was planned to make use of a lightweight engine, and with the saved weight, increase the thickness of the pressure hull - thus allowing for greater diving depths. The project, however, was cancelled.
Little information is available on the Type VIII U-boat, other than it was planned for production in the event of mobilization in 1935.
The Type XI U-boat was planned as an artillery boat; its main armament would have been four 127mm guns, in two twin gun turrets. It would have also carried an Arado Ar 231 collapsible floatplane. Four boats (U-112, U-113, U-114, and U-115) were laid down in 1939, but cancelled at the outbreak of World War II. Had the Type XI U-boat been constructed, it would have had a completely new hull design and a submerged displacement of 4,650 tons – she would have been by far the largest of the U-boats.
The Type XVIII U-boat was a project for an attack boat using the Walter Propulsion System. Two boats (U-796 and U-797) were laid down in 1943, but construction was cancelled in March 1944.
The Type XIX U-boat was a project for an unarmed transport U-boat, based on the Type XB minelayer.
The Type XX U-boat was another project for a transport U-boat based on the Type XB, it would have had a shorter hull than the Type XB, but had a greater beam and draft. 30 Type XX U-boats, were laid down in 1943, but construction stopped in 1944. In August 1944, construction on three Type XX U-boats (U-1701, U-1702, and 1703) resumed, but again stopped in early 1945.