Developed with private funds and operated by the Norddeutscher Lloyd Line, she was one of the first of seven U-151-class U-boats built and one of only two used as unarmed cargo submarines.

Bremen was one of seven submarines designed to carry cargo between the United States and Germany in 1916, through the naval blockade of the Entente Powers.

Five of the submarine freighters were converted into long-range cruiser U-boats (U-kreuzers) equipped with two 10.5 cm (4.1 in) deck guns, and only two were completed according to the original design: Deutschland and Bremen.

Bremen departed Bremerhaven in September 1916 for Norfolk, Virginia, commanded by Kapitänleutnant Karl Schwartzkopf, and reportedly carrying financial credits for Simon Lake to begin building cargo submarines for Germany.

[3] One source records that a submarine believed to have been Bremen was sighted 300 nmi (560 km; 350 mi) south of Iceland on a course for Baltimore and units of 10th Cruiser Squadron were dispatched to intercept it; HMS Mantua reported ramming a heavy, submerged object.