'Bolero' was the Allied cover name for the movement of US forces across the Atlantic to the UK in preparation for the opening of a 'second front' in North-West Europe (1943/44).

The associated deception plan was 'Larkhill'.

The genesis of 'Bolero', which was conceived as an essential preliminary for 'Round-up', lay in the concept for the build-up of US strategic air power in the UK schemed by Major General Henry H. Arnold, the commanding general of the USAAF, and submitted to General George C. Marshall, chief-of-staff of the US Army, on 12 April 1942. This started the vast movement of US men and matériel which paved the way to 'Overlord'.

To complement the 'Round-up' invasion plan, planning for the movement and basing of US forces in the UK was started at the end of April 1942 by a combined committee of British and US logistics specialists in Washington, DC, and London. With 'Round-up' initially scheduled for implementation in April 1943, in May 1942 the Operations Division of the War Department and USAAF Headquarters drafted plans to transport and house 1 million US troops in the form of 525,000 ground troops, 240,000 air force personnel, and 235,000 men of the Services of Supply.

Arnold’s plan was based on the need for the USA to have in the UK, by 1 April 1943, 21 heavy bombardment groups (Boeing B-17 Flying Fortress and Consolidated B-24 Liberator four-engined heavy bombers), eight medium bombardment groups (North American B-25 Mitchell and Martin B-26 Marauder twin-engined medium bombers), nine light bombardment groups (Douglas A-20 Boston twin-engined warplanes), 17 fighter groups (Lockheed P-38 Lightning twin-engined and Bell P-39 Airacobra, Curtiss P-40 Warhawk and Republic P-47 Thunderbolt single-engined aircraft), six observation groups, and eight transport groups, in short 69 combat groups plus their service units.

At the end of May 1942 Arnold presented Air Chief Marshal Sir Charles Portal, the Chief of the Air Staff, with the schedule for the arrival of USAAF forces in the UK by March 1943, totalling 3,649 aircraft. The proposed build-up anticipated 15 groups in July, 35 by November and 66 by March, excluding observation squadrons. Arnold anticipated that by 1 April the combat units of the US 8^th AAF would have 800 heavy bombers, 600 medium bombers, 342 light bombers and 960 fighters, when at this moment the US 8^th AAF in the UK was 1,871 personnel and no aircraft.

The movement of the relevant air combat groups began in May 1942 with the shipment of their ground echelons by fast troopship. The movement of their aircraft began in June after it had been decided that the most efficient and rapid build-up could be made by having the groups themselves fly their aircraft across the North Atlantic. With only the pilot and no navigational or communications equipment for transoceanic flights, the fighters were to be divided into flights of four and escorted by single bombers providing the required navigation and communication link.

Three groups were assigned for the first phase of movement: the 97^th Bombardment Group with B-17 Flying Fortress aircraft, the Fighter Group with P-38 Lightning aircraft, and the 60^th Troop Carrier Group with Douglas C-47 Skytrain twin-engined transport aircraft. These were gathered on the eastern coast of the USA in the 'concentration area' to stage for the overseas flights via the northern ferry route. All of these groups, originally allocated to the US 8^th AAF, saw limited service in the UK before being reallocated to the US 12^th AAF.

Although a loss rate of 10% for the first movement had been thought likely, the actual rate was in the order of 5%. The largest loss was on 27 June 1942, when six P-38 fighters of the 1^st Fighter Group and two B-17 bombers of the 97^th Bomb Group, on the leg of the transatlantic ferry route between Greenland and Iceland, ran into averse weather and were compelled to turn back toward Greenland: running short of fuel, all eight aircraft force-landed on the Greenland ice cap, and though the aircraft were abandoned their crews were rescued.

By the end of August 1942, the 'Bolero' undertaking had seen the arrival in the UK of 386 aircraft in the form of 164 P-38 machines of the 1^st and 14^th Fighter Groups, 119 B-17 machines of the 97^th, 301^st and 92^nd Bomb Groups, and 103 C-47 machines of the 60^th and 64^th Troop Carrier Groups. By the end of 1942 the tally of aircraft which had reached the UK was 882 machines out of the 920 which had set off from the USA, almost all of them delivered by their combat crews rather than specialised ferry command personnel.

By 1 July the demands of other theatres for air reinforcement had forced the USAAF to scale back its ambitions for 'Bolero' to 54 groups and 194,332 men. Later that month USAAF Headquarters estimated that by 31 December 1943, the 'Bolero' build-up could deliver 137 groups, which was about half of the USAAF’s projected strength, including 74 bombardment groups of all types and 31 fighter groups with some 375,000 personnel (197,000 in combat units and 178,000 in the service organisations). The estimate proved to be remarkably close, particularly the size of the heavy bomber force, to the actual strength of the US 8^th and 9^th AAFs at the time of 'Overlord'.

In London the 'Bolero' Committee drew up plans for the accommodation of 1.147 million troops, including 137,000 replacements, in the UK by the end of March 1943. But by the end of July, the plan for the 'Sledgehammer' invasion of Europe had been abandoned for 'Torch' landings in North-West Africa, and 'Round-up' was postponed until at least 1944. The 'Bolero' Committee thus found its work limited to providing assistance in the planning for an invasion of North Africa. Build-up plans for the invasion of Europe later became the province of the 'Overlord' planners.