Valiant V bomber in 1961.
The RAF Bomber Command ended World War II with a policy of using heavy four-piston-engined bombers for massed raids, and remained committed to this policy in the immediate postwar period, adopting the Avro Lincoln, an updated version of the WW2 Lancaster, as their standard bomber.
The development of jet aircraft and nuclear weapons soon made this policy obsolete. The future appeared to belong to jet bombers that could fly at high altitude and speed, without defensive armament, to perform a nuclear strike on a target. Although, even at the time there were those who could see that guided missiles would at some time in the future make such aircraft vulnerable, but development of such missiles was proving difficult, and fast and high-flying bombers were likely to serve for years before there was a need for something better.
In any case, massed bombers were unnecessary if a single bomber could destroy an entire city or military installation with a nuclear weapon. It would have to be a large bomber, since the first generation of nuclear weapons were big and heavy. Such a large and advanced bomber would be expensive on a unit basis, but would also be produced in much smaller quantities. Britain had been economically bled dry by World War 2, and the potential economies were attractive.
The arrival of the Cold War also emphasized to British military planners the need to modernize British forces. Furthermore, Britain's up-and-down relationship with the USA, particularly in the immediate postwar years when American isolationism made a short-lived comeback, led the British to feel they needed their own strategic nuclear strike force.
After considering various specifications for such an advanced jet bomber in late 1946, in January 1947 the Air Ministry issued an request for an advanced jet bomber that would be at least the equal of anything the US or the USSR had. The request followed the guidelines of the earlier Specification B.35/46, which proposed a "medium-range bomber landplane, capable of carrying one 10,000 pound [4,535 kilogram] bomb to a target 1,500 nautical miles [2,775 kilometers] from a base which may be anywhere in the world."
The request also indicated that the fully loaded weight not exceed 45,350 kilograms (100,000 pounds), though this would be adjusted upward in practice; that the bomber have a cruise speed of 925 km/hr (500 knots); and that it have a service ceiling of 15,240 meters (50,000 feet).
The request went to most of England's major aircraft manufacturers. Handley Page and Avro came up with very advanced designs for the bomber competition, which would become the Victor and the Vulcan respectively, and the Air Staff decided to award contracts to both companies, again as a form of insurance. While the Vickers-Armstrong's submission had been rejected as too conservative, Vickers' lobbied the Air Ministry and made changes to meet their concerns and managed to sell the Vickers design on the basis that it would be available much sooner than the competition, and would be useful as a "stopgap" until the more advanced bombers were available. Apparently the Air Ministry didn't think there could be too much insurance.
The development of effective anti-aircraft missiles made the deterrent threat increasingly threadbare. After the failed Blue Streak missile program and the cancellation of the American Skybolt and British Blue Steel Mk. 2, the long-serving Vulcans were displaced in the strategic role by the Polaris missile which would have been launched from submarines.