http://www.onwar.com/chrono/1940/mar40/f31mar40.htm
German armed merchant cruiser sails
Sunday, March 31, 1940 www.onwar.com
In the North Atlantic... The first German armed merchant cruiser, Atlantis, sails for operations against Allied shipping. It will prove to be the most successful raider. In a cruise lasting until November 22, 1941 she will sink 22 ships of 145,700 tons.
The Germans had used adapted merchantmen in the First World War, initially as minelayers and then as auxiliary cruisers (Hilfskreuzer). When their conventional warships and then the auxiliary cruisers had been swept from all the world's oceans except the North Sea, they turned to the disguised raider, with which they scored some spectacular successes even as late as the beginning of 1918. Inspired by this example and mindful of the tiny conventional raiding-forces at his disposal, Raeder planned before the war to arm twenty-five suitable merchant-men with a main armament of 5.9inch guns plus torpedoes, AA- and machine-guns. In the end only ten were completed, of which the first six sailed in spring 1940, starting with the Atlantis on March 31. She proved to be the most successful, sinking nearly 150,000 tons of shipping (three times as much as the Graf Spee and seven times the haul of the Deutschland) before she was sunk by HMS Devonshire twenty months later.
- Dan Van der Vat, The Atlantic Campaign, pp 178-179.