Being so big and heavy, Blue Danube could only be carried by the V bombers, so-called because they all had names starting with a "V". The three strategic bombers, known collectively as the V class, comprised the United Kingdom's strategic nuclear strike force during the 1950s and 1960s, which was known as V force of the Main Force. The three V bombers were the Vickers Valiant, which entered service in February 1955; the Avro Vulcan, which entered service in May 1956; and the Handley Page Victor, which entered service in November 1957. The V Bomber force reached its peak in June 1964, when 50 Valiants, 70 Vulcans and 39 Victors were in service.