The Valentin submarine factory is a protective shelter on the Weser River at the Bremen suburb of Rekum [de; nds], built to produce and launch German U-boats during World War II.

Other sites in Germany and other occupied countries were under construction or planned, such as Hornisse in Bremen, Elbe XVII and Wenzel in Hamburg, Wespe in Wilhelmshaven, Kaspar in Kiel.

Under the codename Valentin a submarine factory was to be built directly on the Weser river between the Bremen suburbs Rekum and Farge.

The roof was constructed using dozens of large, reinforced concrete arches, manufactured on-site and individually lifted into place.

The two separated bays could be flooded to give a total water depth of about 20 metres from the bottom of Takt 13's dry dock to the water-surface at the building's roof-level.

In addition to the 13 assembly bays, the bunker housed workshops and store-rooms for the prefabricated sections, diesel-engines and batteries, and storage tanks for fuel and lubricants.

The gateway in the western wall could be closed by means of a sliding bomb-proof door which opened to a small canal, a creek and then directly onto the Weser river.

[1] Operations at Valentin were intended to commence by late 1944, but was postponed to mid-1945 due in part to a combination of manpower and supply shortages and bombing.

Albert Speer (the Reich armaments minister) had directed that the sections be made by inland companies and then assembled at the shipyards so as to ease production.

A French survivor, Raymond Portefaix, stated that a prisoner's life expectancy fell dramatically on being assigned to one of these detachments.

[6] The prisoners held at the Neuengamme concentration camp and its subcamps were evacuated in April 1945, just before the capture of the area by the British army.

The attacking force consisted of twenty Avro Lancaster heavy bombers of 617 Squadron which had, after the "Dambusters" raid, developed precision bombing methods.

Post–war American analysis suggested the 7-metre (23 ft) thick, east section would have been able to resist even the Grand Slams, although not without significant damage, and it is unlikely that it would have survived repeated hits.

The factory was abandoned, and four weeks after the bombing, the area was occupied by the British Army's XXX Corps, which captured Bremen after a five-day battle.

Beginning in March 1946, Project Ruby was a joint Anglo-American affair to investigate the use of penetration bombs against heavily protected concrete targets.

[1] The Valentin-bunker was bombed with inert (non-explosive) loads to protect nearby buildings and civilians, to test things like penetration capability and strength of the bomb-shell during impact.

The U-boat pen Nordsee III and subterranean bunkers on the uninhabited island of Heligoland were subjected to explosive-loaded bombs to study aspects of huge explosions on massive concrete buildings.