Born in Sault Ste. Marie, Ontario, Duncan debuted in professional hockey in the 1915–16 season. Duncan's October 1, 1916 enlistment papers gave his occupation as accountant. He listed his next of kin as his mother, Mrs. W. A. Duncan. He listed his home address as his mother's place in Toronto. He also claimed to be serving in the 34th Regiment of the militia, which may be the basis for his direct commissioning as an officer. During the 1916 and 1917 hockey seasons, Duncan continued to play, as his battalion fielded a team in the National Hockey Association both years. His transfer to France cut off his sporting career.
World War I
Duncan first served with the 228th Battalion CEF in France as a lieutenant. On August 1, 1917, Lieutenant W. J. A. Duncan of the Canadian Railway Troops was appointed a Flying Officer in the Royal Flying Corps and seconded for duty with them. He was transferred to the Royal Flying Corps and assigned to 60 Squadronin September 1917 as a Royal Aircraft Factory SE.5a pilot with a roving commission. Duncan scored his first aerial victory on November 6, 1917, when he destroyed a German DFW reconnaissance plane northeast of Polygon Wood. He began to accumulate wins. On the day the RFC was consolidated into the Royal Air Force, April 1, 1918, he shared his seventh victory with American ace John Griffith. On May 17, 1918, he raised his tally to nine. The next day, he was promoted to temporary captain. He would score twice more after that, destroying an LVG on June 3 and busting a German observation balloon on June 5, 1918. He was returned to Home Establishment on June 30, 1918. A summary of his victories included an enemy observation balloon destroyed, an enemy fighter plane captured, seven enemy planes destroyed, and two enemy airplanes driven down out of control. His combat exploits won him the award of the Military Cross, gazetted on July 26, 1918:
For conspicuous gallantry and devotion to duty. On one occasion he attacked and shot down an enemy plane which had been engaged at firing on our infantry. He then led his patrol over the enemy's lines, dived down to an altitude of 100 feet, and attacked large numbers of hostile infantry with machinegun fire, causing the utmost panic amongst them and inflicting heavy casualties. His continuous gallantry and initiative have been most conspicuous.
An award of a Bar to the Military Cross in lieu of a second award followed on September 16, 1918:
For conspicuous gallantry and devotion to duty. This officer sighted fifteen enemy scouts attacking eight of ours and immediately joined in, destroying one enemy aeroplane, which fell with a wing off. He then attacked and drove down three other machines, maintaining the fight until the eight had got back to their lines. He has also, with another officer, destroyed an Albatros scout, which he followed down to a height of 200 feet, in spite of heavy machine-gun fire from the ground.
