One of the most important weapons of marine warfare was the mine. In order to bring this weapon close to the enemy shipping routes, the German Navy used submarines. The U-Boot Type VIID was a lengthened sub-variant of the successful type VIIC combat version. In order to be able to transport and lay type SMA anchor rope mines, a section with five mine silos each carrying 3 SMA mines was built in behind the conning tower. This increased the overall length by 9.80 m.