On 1 January 1944, Grenier Field became the headquarters of the North Atlantic Division of Air Transport Command (ATC).  Its primary mission was the ferrying of aircraft from the United States to Great Britain. Using airfields at Presque Isle, and Dow, Maine, ATC ferried thousands of combat and non-combat aircraft through airfields in Newfoundland and Labrador through Greenland and Iceland to Scotland. Long-range transports could also fly directly across the Atlantic to England or via the Azores Islands. The Air Weather Service established a detachment at Grenier which provided long-range weather reconnaissance over the North Atlantic convoy shipping lanes.

Grenier Field became involved in a bold Army Air Force plan known as Bolero in June, 1942. Bolero was a code name for the rapid buildup of American combat forces in England. Wartime transatlantic crossings by multi-engine aircraft had become routine, but the supply of fighter aircraft to England was a different matter. The most common method of crossing had been as deck cargo aboard surface vessels. Aside from being prohibitively slow and risky, this method exposed the relatively delicate airframes to manhandling and the corrosive effects of saltwater. An innovative plan was developed whereby the fighters would be ferried across the ocean in a series of short hops with navigational assistance provided by bombers. Most Bolero fighters departed from Presque Isle AAF, Maine, but during June 1942, the 52nd Fighter Group began to work up with Bell P-39 Airacobras at Manchester. Army Air Force leaders soon decided the P-39 was not an ideal fighter for the European theater, and the personnel of the 52nd went to England by ship. It wasn't until late in the year, after being reequipped with the legendary Spitfire fighter, that the 52nd began to operate from England as a combat fighter group.