A Yank in the RAF

In 1940, American-built North American Harvard training aircraft are flown to just outside Canada, where they are towed across the border for use by Britain. (The procedure is necessary to avoid violating the Neutrality Acts, as the United States is still neutral.) Cocky American pilot Tim Baker (Tyrone Power) decides to fly across the border to Trenton, Ontario, and winds up in trouble with the military authorities, unconvincingly claiming he was looking for Trenton, New Jersey. Baker ferries a Lockheed Hudson bomber to Britain, pocketing $1,000 for his work.In London, he runs into his on-again off-again girlfriend Carol Brown (Betty Grable), who works in the Women's Auxiliary Air Force by day and stars in a nightclub by night. She is none too pleased to see him, calling him a "worm" for his womanizing ways, lying, and long absence, but he is confident she still harbors strong feelings for him.He decides to enlist in the Royal Air Force (RAF). Meanwhile, Brown attracts the appreciative attention of two RAF officers, Wing Commander John Morley (John Sutton) and Flying Officer Roger Pillby (Reginald Gardiner). Morley persists in seeing Brown, despite being told at the outset that there is another man. Pillby is unable to persuade either Baker or Morley to introduce him.After completing training, Baker is disappointed to be assigned to Morley's bomber squadron, rather than to a fighter. He becomes further disgruntled when his first mission is to "bomb" Berlin with propaganda leaflets as Morley's co-pilot during the Phoney War. Pillby pilots another bomber in the raid.When Baker is late for their date (sidetracked by meeting an old buddy from America), Brown accepts Morley's invitation to spend a weekend at his country estate. There, Morley asks her to marry him. When she tells Baker about it (without revealing who her suitor is), he offers to marry her himself, but in an insultingly casual way. She tells him that they are through. Back at the base, the two rivals learn of each other's involvement with the same woman. Before they can do anything about it, however, the Germans invade Holland and Belgium, and they are given an urgent mission to bomb Dortmund, Germany, this time with real ordnance.During the nighttime raid, their bomber is hit, disabling one of their two engines. Pillby descends to their aid, knocking out searchlights, but is shot down in flames and perishes. Morley orders his crew to bail out, but Baker disobeys and lands the aircraft on a Dutch beach. Spotting a line of German soldiers, they hide in a nearby building, only to be taken prisoner by a German officer there. A crewman sacrifices himself, enabling the other two to dispatch the German and escape by motorboat.Baker wakes up in a British hospital, the victim of exposure. Once discharged, he goes to see Brown, pretending to have a broken arm, but blunders and shows himself to be a liar once more. Nonetheless, he produces an engagement ring and forces it onto her finger. After receiving a telephone call from Morley breaking their date, Brown informs Baker that all leaves have been canceled.Reserves are called up to make up fighter pilot losses, and Baker is reassigned to a Spitfire for the Battle of Dunkirk. He downs two Luftwaffe fighters before being shot down. Carol cannot hide her distress when she cannot find out whether he is alive or not. Morley takes her to the docks, where ships returning from the Dunkirk beaches are bringing back survivors. When Baker debarks, Carol rushes to him and shows him she is still wearing his ring.

Question: why are the planes towed across the border
Answer: to avoid violating the Neutrality Acts