The idea of merchant submarines, a variation on standard merchant ships, has been suggested on multiple occasions, but has only been put into practice once, during WWI. In 1916 a class of seven u-boats was built by a private shipping company. These new u-boats were large, displacing over 2,200 tons, and with a wide beam to facilitate loading. They had no torpedo tubes or other armament, being civilian vessels operated by a civilian company, the North German Lloyd Line.
Although they were large by u-boat standards, their cargo capacity of around 700 tons was relatively small by surface ship standards. Their ability to submerge to avoid detection, however, was a significant advantage for a country that was under an efficient naval blockade. The blockade, put in place by the Entente powers, was severely hampering German trade and making it difficult for German companies to acquire raw materials. Of the seven merchant submarines built, only two were used in their intended role, and of those two, SMS Bremen sank on her maiden voyage. SMS Deutschland made two successful round voyages to the United States.