the submarine was laid down on 20 november 1937 as "k xxi" at the "koninklijke maatschappij de schelde", vlissingen during construction she was renamed "o 21", and was finally launched on 21 october 1939. following the german invasion of 10 may 1940, the "o 21" was hastily commissioned, still incomplete, and sailed for england on 12 may together with her sister and the tugboat "b.v. 37", to be finally completed at the "navy yard" in rosyth.