This title follows on from Volume I and charts the continuing development of the U-boat in German service. This includes the development of the Type IX as a long range 'cruiser' intended for solo operations in distant waters. Also covered is the revolutionary Type XXI, conceived of in 1942 and launched in April 1944, the first true submarine rather than submersible, whose arrival was just too late to influence the war. Other vessels included are the Type XXIII, small and armed with only two torpedoes but technically highly advanced, and the Type X minelayers. These were rarely used in their intended role and were more often used as supply boats.