German submarine U-20 was a Type IIB U-boat of Nazi Germany's Kriegsmarine. Her keel was laid down on 1 August 1935, by Germaniawerft of Kiel as yard number 550. She was commissioned on 1 February 1936. During World War II, she conducted operations against enemy shipping.

The 1st U-boat flotilla also known as the Weddigen flotilla, was the first operational U-boat unit in Nazi Germany's Kriegsmarine (navy). Founded on 27 September 1935 under the command of Fregattenkapitän Karl Dönitz, it was named in honor of Kapitänleutnant Otto Weddigen. Weddigen, a U-boat commander during World War I, died on 18 March 1915 after his submarine U-29 was rammed by the British battleship HMS Dreadnought in the North Sea.