They were an improvement over previous designs, which suffered from insufficient range to be effective commerce raiders. However, the requirements placed on the design—high maximum and cruising speeds, long range, heavy armament, and armor sufficient to withstand 15 cm shells, all on a displacement no more than 8,000 metric tons—were deemed impossible by the design staff. In July 1937, the Oberbefehlshaber der Marine (commander in chief of the navy) requested proposals from both the naval design staff as well as private dockyards.