Type X (XB) U-boats were a special type of submarine (German U-boat) intended as Mine-layers, but later used as long range cargo transports. A task they shared with the Type IX-D1 and Italian Romolo class submarines.

These XB boats were laid down as ocean-going submersibles, designed in 1938. They could carry up to 66 SMA mines in 30 vertical mine shafts and up to 15 torpedoes. The XB class was special in that they only had two torpedo tubes at the stern. As transport boats they carried freight containers in the mine shafts (or had the freight containers welded on top of the lateral shafts, preventing their use for mines). At 2710 tons submerged and fully loaded they were the largest German U-boats ever built and they paid a penalty in diving speed and agility.

Six of the 8 boats built were sunk during the war (5 with all hands) but two survived the war. One survivor was U-234, which was infamous for a 560kg cargo of Uranium-oxide which it had en-route for Japan. Other cargo included two Me-262 jet fighters and 10 jet engines. The other type XB to survive was U-219 which reached Djakarta in December 1944 with a cargo including dismantled V-2 rockets for Japan.U-234 U-219