Traill joined the
Royal Navy as a
midshipman on 2 August 1914, when he was just four days shy of his 15th birthday. He was assigned to Traill transferred to the
Royal Flying Corps to train as a pilot, and after completion of training was
commissioned as a temporary
second lieutenant on probation on 11 October 1917. He was assigned to
No. 20 Squadron RFC that day as a
Bristol F.2 Fighter pilot. Traill was promoted to
lieutenant on 1 April 1918, as the
Royal Air Force came into existence. He was promoted to temporary
captain when he was appointed as a
flight commander on 28 September 1918. Traill scored eight aerial victories. In the process, he had three other aces serve as his gunner/observer at various times. While in combat on 2 July 1918, Percy Griffith Jones called out a warning from the plane's rear seat and Traill ducked. The German fighter behind them killed Jones and put a bullet through the cockpit and out the windscreen, missing Traill. Traill's next observer took an incendiary bullet in his leg.
Leslie William Burbidge then became Traill's observer. On 23 October, while returning from the mission upon which Traill scored his eighth victory, Traill collided with another plane in his flight while flying at 7,000 feet. The accident knocked away part of the Bristol F.2 Fighter's wing. As the fighter tried to spin out of control, Burbidge leaped out onto the opposite wing at Traill's command, to counterbalance the spin while Traill struggled for control. The resultant crashlanding hurled Burbidge onto his face, but left Traill uninjured and preserved the airplane. Both men were awarded the
Distinguished Flying Cross for this incident. Traill's citation read ==List of aerial victories==