Air Ministry Operational Requirement OR.1177[1] was issued for a new design using a less sensitive explosive, which was undertaken at the Atomic Weapons Research Establishment as "Cleo".

When Skybolt was cancelled, the UK gained access to the UGM-27 Polaris missile and its W58 warhead, but they continued development of Cleo as a tactical weapon to replace Red Beard.

In May 1960, Prime Minister Harold Macmillan signed an agreement with President Eisenhower to purchase 144 AGM-48 Skybolt missiles for the UKs V bomber force.

Katie would be used in a new bomb casing to produce WE.177A, replacing Red Beard with a weapon of roughly 1/3 the weight, and much smaller size.

WE.177 was adapted to produce a high-yield interim strategic weapon for the five-year period, while the Polaris submarine force was building.

Halting work on the original WE.177, now known as the 'A' model, a new version that used the W59 secondary, codenamed Simon, matched with a modified 'Katie B' primary created WE.177B.

Type A, B and C weapons were carried by strike aircraft, including the Avro Vulcan,[2] de Havilland Sea Vixen, Blackburn Buccaneer,[2] SEPECAT Jaguar,[2] and Panavia Tornado.

As with all British thermonuclear weapons, the tritium gas used in the bomb core was purchased from the United States as part of the 1958 US–UK Mutual Defence Agreement; that permitted the US to obtain UK weapons-grade plutonium, in exchange for enriched uranium, tritium, and other specialised material uneconomical to produce in the UK in the very small quantities required.

An industrial plant codenamed Candle located adjacent to the Chapelcross nuclear power station, near the town of Annan, Dumfries and Galloway, Scotland, was built to recover tritium from time-expired service weapons returned for routine maintenance or servicing.

Part of the safety and arming system on the WE.177 series was a simple key-operated Strike Enable Facility; using a cylindrical barrel key similar to those used on vending or gaming machines.

By agreement with the owners of the lock's design rights, the key profile for each and every live weapon was unique, and would not be used for any other purpose.

The casing of WE.177 was unusually robust, and complicated, for a British air-dropped bomb; made necessary by the requirement for the laydown delivery[3] options.

This, together with the 'slap down' of the tail on impact required a strong, well-engineered bomb casing to ensure the enclosed warhead remained intact.

[2] At least forty-three were deployed aboard Royal Navy surface vessels of frigate size and larger; for use by embarked helicopters as an anti-submarine nuclear depth bomb,[4] starting in 1971.

Helicopter-delivered nuclear depth bombs were not always immediately available, due to fuel-state, other taskings, or expended weapons load.

When the Navy's large aircraft carriers were decommissioned, around twenty warheads were transferred to the Royal Air Force.

Numbers built are still uncertain, but reliable sources put the figure at fifty-three (53), and all were retired by August 1998.

[2] When Polaris became operational, the Vulcan force continued in a sub-strategic tactical role with these and other bombs assigned to the NATO SACEUR.

[11] A Mk.8 torpedo was chosen to sink the Argentinian warship General Belgrano, because it was of proven reliability, unlike the unreliable Tigerfish.

In 1982, with the outbreak of the Falklands War, some of these vessels were urgently assigned to the Naval Task Force, and began to steam south with their nuclear weapons still on board.

[4] The Ministry of Defence (MoD) has said that, en route, the bombs were offloaded from escort vessels Broadsword, Brilliant, Coventry and Sheffield; and were stored in the better-protected deep magazines aboard Hermes and Invincible; and the fleet replenishment ships Fort Austin, Regent, Resource, and Fort Grange who were accompanying the Task Force.

[4] The MoD assert that the ships carrying nuclear depth bombs did not enter Falkland Islands territorial waters, or any other areas subject to the Treaty of Tlatelolco[4] (that established the Latin America Nuclear Weapons Free Zone), to which the UK was a signatory.

[4] They also state that all the nuclear weapons were returned to the UK aboard the Royal Fleet Auxiliaries Fort Austin and Resource on 29 June and 20 July 1982 respectively, after the end of the Falklands War.

[14] Trident D5 is the UK's sole remaining nuclear weapons delivery system (see Vanguard-class submarine), believed armed with a strategic warhead also usable in the sub-strategic role formerly performed by WE.177.