The Type VII was a class of medium attack U-boats built for Nazi Germany's Kriegsmarine from 1935 to 1945. Derived from the World War I design of the Type UB III and the Vetehinen class built for Finland, the Type VII was designed for attacking the North Atlantic convoy lanes and formed the backbone of the German effort in the Battle of the Atlantic during World War II. The Type VII came in several subtypes: The first subtype VIIA had some shortcomings in handling, range and armament, and these shortcomings were addressed in the subsequent VIIB. When more space was needed for a sonar, a slightly enlargened version VIIC was put into production. The fourth subtype VIIC/41 featured a strengthened pressure hull for improved diving depth. Further subtypes included a mine laying Type VIID and a torpedo supply U-boat Type VIIF.