 She was launched on 16 February 1939 and commissioned on 15 July of that same year under the command of "Kapitänleutnant" Rolf Dau.As one of the eight original German Type IX submarines, later designated IXA, "U-42" had a displacement of when at the surface and while submerged.The U-boat had a total length of , a pressure hull length of , a beam of , a height of , and a draught of .