The first practical design of a submarine-based launch platform was developed by the Germans near the end of World War II involving a launch tube which contained a V-2 ballistic missile variant and was towed behind a submarine, known by the code-name "Prüfstand XII". The war ended before it could be tested, but the engineers who had worked on it went on to work for the United States and for the Soviet Union on their SLBM programs. These and other early SLBM systems required vessels to be surfaced when they fired missiles, but launch systems eventually were adapted to allow underwater launching in the 1950-1960s. A converted Project 611 (Zulu-IV class) submarine launched the world's first SLBM, an R-11FM (SS-N-1 Scud-A, naval variant of the SS-1 Scud) on 16 September 1955. Five additional Project V611 and AV611 (Zulu-V class) Submarines became the world's first operational ballistic missile submarines (SSBs) with two R-11FM missiles each, entering service in 1956–57.