Fullard joined the Inns of Court Officer Training Corps in 1915, and after receiving high marks in his examination was offered a commission in the Royal Irish Fusiliers, but was then selected for the Royal Flying Corps. He was commissioned as a probationary temporary second lieutenant for duty with the RFC on 5 August 1916. He trained at the No. 2 School of Military Aeronautics at Oxford, with No. 3/24 Reserve Squadron, and at the Central Flying School at Upavon, and on 26 December was appointed a flying officer and confirmed in his rank as a second lieutenant on the General List. Fullard soloed after only three hours flying time, and on account of his exceptional flying ability, he was retained as an instructor at Upavon, but was eventually sent to serve in No. 1 Squadron RFC in France in May 1917.