Why did many Vulcans have bulges for a missile they never used?

In the late 1950s the United Kingdom was looking for a weapon that would let its bombers attack without getting too close to enemy defences. The Vulcan B.2 could already carry the Blue Steel, a stand-off missile developed by Avro, and 33 aircraft had been delivered with the necessary modifications. The improved version, the Blue Steel Mk.2, was cancelled in December 1959 in favour of another weapon: the US-developed Douglas GAM-87 Skybolt air-launched ballistic missile.

To carry the new missile, the Vulcans had their fittings changed: the Skybolt would be carried under the wings, one on each side. Many aircraft were delivered from the factory, or converted, with the corresponding bulged fairings, known as "Skybolt blisters".

Skybolt was cancelled in November 1962, on the recommendation of US Defense Secretary Robert McNamara, and the decision triggered the so-called Skybolt crisis between Washington and London. The RAF kept the Blue Steel. The fairings, however, remained on many aircraft as a reminder of a weapon they never got to carry.

Afterwards, the deterrence role passed to the Royal Navy with its Polaris missile submarines, and the Vulcan switched to low-level tactics. Avro even proposed a Vulcan that would carry three Gnat fighters under the fuselage to escort itself, an idea that never went ahead.

The aircraft originated from the British Ministry of Supply's B.35/46 specification. The first prototype flew on 30 August 1952, and the RAF received its first Vulcan B.1 in September 1956.