Project Emily was the deployment of American-built Thor intermediate-range ballistic missiles (IRBMs) in the United Kingdom between 1959 and 1963. Royal Air Force (RAF) Bomber Command operated 60 Thor missiles, dispersed to 20 RAF air stations, as part of the British nuclear deterrent.

Due to concerns over the buildup of Soviet missiles, US President Dwight D. Eisenhower met Prime Minister Harold Macmillan in Bermuda in March 1957 to explore the possibility of short-term deployment of IRBMs in the United Kingdom until the long-range intercontinental ballistic missiles (ICBMs) were deployed. The October 1957 Sputnik crisis caused this plan to be expedited. The first Thor missile arrived in the UK on a Douglas C-124 Globemaster II transport aircraft in August 1958, and was delivered to the RAF in September.

RAF crews periodically visited the United States for training, culminating in 21 operational training launches from Vandenberg Air Force Base. During the Cuban Missile Crisis in October 1962, 59 of the missiles, with their W49 1.44-megaton-of-TNT (6.0 PJ) thermonuclear warheads, were brought to operational readiness. The Thor missile force was disbanded in 1963, and the missiles were returned to the United States, where most were expended in military space shots.

During the early part of the Second World War, Britain had a nuclear weapons project, codenamed Tube Alloys. At the Quebec Conference in August 1943, the Prime Minister of the United Kingdom, Winston Churchill and the President of the United States, Franklin Roosevelt, signed the Quebec Agreement, which merged Tube Alloys with the American Manhattan Project to create a combined British, American and Canadian project. The British government trusted that the United States would continue to share nuclear technology, which it regarded as a joint discovery, after the war, but the United States Atomic Energy Act of 1946 (McMahon Act) ended technical cooperation. Its control of "restricted data" prevented the United States' allies from receiving information related to nuclear weapons. Fearing a resurgence of United States isolationism, and Britain losing its great power status, the British government restarted its own development effort, which was codenamed High Explosive Research. The first British atomic bomb was successfully detonated off the Monte Bello Islands in Operation Hurricane on 3 October 1952, and the first production model atomic bomb was delivered to the Royal Air Force (RAF) in November 1953.

Britain's nuclear weapons armament was initially based on free-fall bombs delivered by the V bomber force, but the possibility of the bomber becoming obsolete by the late 1960s was foreseen. In 1953, the Assistant Chief of the Air Staff (Operational Requirements), Air Vice Marshal Geoffrey Tuttle, asked for a specification for a ballistic missile with 2,000-nautical-mile (3,700 km) range to be drawn up ahead of design work commencing. This became Operational Requirement OR.1139. Work commenced at the Royal Aircraft Establishment in Farnborough later that year. In March 1954 the Ministry of Supply was asked to put forward proposals for a full-scale ballistic missile project. At a NATO meeting in Paris in December 1953, the United States Secretary of Defense, Charles E. Wilson, raised the possibility of a joint development programme with the Minister of Supply, Duncan Sandys. Talks were held in June 1954, resulting in the signing of an agreement on 12 August 1954. The United States would develop an intercontinental ballistic missile (ICBM) of 5,000-nautical-mile (9,300 km) range, while the United Kingdom with United States support would develop a medium-range ballistic missile (MRBM) of 2,000-nautical-mile (3,700 km) range. The American ICBM was codenamed Atlas, while the British MRBM was called Blue Streak. Blue Streak was estimated to cost £70 million, with the United States paying 15 per cent of the cost.

In parallel to the ICBM programme, the United States developed three separate intermediate range ballistic missile (IRBM) systems. On 8 November 1955, Wilson approved IRBM projects by both the United States Army and the United States Air Force (USAF). The United States National Security Council gave the ICBM and IRBM projects the highest national priority. The Army Ballistic Missile Agency, commanded by Major General John B. Medaris, with Wernher von Braun as its technical director, developed the Jupiter (SM-78) IRBM. After three failures, its first successful flight took place at the Cape Canaveral Air Force Station on 31 May 1957. The United States Navy initially participated in the Jupiter programme, intending to launch the missiles from ships, but in view of their size, decided instead to develop a smaller, solid-propellant IRBM that could be launched from a submarine, which became Polaris. Despite reluctance on the part of Major General Bernard Schriever, the commander of the USAF Western Development Division, to take on IRBM development, and a pronounced lack of enthusiasm for missiles among senior USAF officers, who preferred manned bombers, the rival USAF project moved quickly. The Douglas Aircraft Company was awarded the contract for the development of the missile, which was codenamed Thor (SM-75), in December 1955. Rocketdyne, Ramo-Wooldridge, AC Spark Plug, Bell Laboratories and General Electric were engaged as the subcontractors for the rocket engine, technical coordination, inertial guidance system, radio guidance system, and re-entry vehicle respectively. The first missile was delivered to Patrick Air Force Base on 26 October 1956. After four unsuccessful attempts, the first successful test flight took place on 20 September 1957.

Implicit in Wilson's decision to develop an IRBM was that it would be based overseas. Thor had a range of 1,500 nautical miles (2,800 km; 1,700 mi), and therefore did not have the range to reach the Soviet Union and China from the United States. Britain, Germany, Turkey, the Philippines, Taiwan, South Korea and Japan were considered as possible sites for deploying Thor IRBMs. Gordon Gray, the US Assistant Secretary of Defense for International Security Affairs, mentioned it to a Ministry of Supply official in January 1956, and unofficial, low-level approaches began in March 1956. The United States Secretary of the Air Force, Donald A. Quarles, officially raised the matter with the Minister of Defence, Sir Walter Monckton, and his Chief Scientific Advisor, Sir Frederick Brundrett, in meetings at the Palace of Westminster on 16 and 17 July 1956. Quarles proposed sites in England for the missiles, and suggested that live firings be conducted from Scotland. Monckton noted that this option had already been rejected for Blue Streak, in favour of test firings at the Woomera Test Range. Monckton and Brundrett considered whether Thor or Jupiter would have the range for British purposes. The Americans could not supply nuclear weapons under the McMahon Act. The missiles could be fitted with British warheads, but these would be heavier, reducing the range to 1,250 nautical miles (2,320 km; 1,440 mi). Brundrett considered such a missile "useless". The Chairman of the Chiefs of Staff Committee, Marshal of the Royal Air Force Sir William Dickson, queried whether Blue Streak was redundant if the Americans supplied Thor. Like many senior RAF officers and their USAF counterparts, he was also concerned about the fate of the manned bomber if the government embraced missile technology.

During the initial negotiations, the major British concerns were about the technical aspects of the weapons, and the costs and benefits of deployment in the United Kingdom rather than concerns about control. A precedent here was Project E, under which data on US nuclear weapons was supplied to Britain to allow English Electric Canberra bombers and V-bombers to carry them in wartime. The McMahon act was amended in August 1954 to permit this. Under Project E, stocks of US nuclear weapons for British use were held at RAF airbases under US custody. On 12 December 1956, the Chief of Staff of the United States Air Force, General Nathan Twining, suggested to his British counterpart, Air Chief Marshal Sir Dermot Boyle, that the Thor missile warheads be made available "under the same terms and conditions". Sandys, now the Minister of Defence, affirmed that this arrangement would be acceptable to the British government. When he visited the United States in January 1957, he found the Americans eager to deploy IRBMs in Britain. The Prime Minister, Harold Macmillan, and President Dwight D. Eisenhower agreed to do so at a summit in Bermuda in March 1957. Although the IRBM negotiations had commenced before the Suez crisis damaged Britain's relationship with the United States, it suited the British government to portray the IRBM deal as a demonstration that the rift had been healed.