Plan Z
"U-42" was ordered by "Kriegsmarine" on 21 November 1936 (as part of Plan Z). Her keel was laid down on 21 December 1937 by DeSchiMAG AG Weser of Bremen as yard number 947. She was launched on 16 February 1939 and commissioned on 15 July of that same year under the command of "Kapitänleutnant" Rolf Dau.