After training as a cadet in the
Officers' Training Corps, Rayner was commissioned as a second lieutenant (on probation) on 14 April 1915, serving in the 1st Battalion,
Northumberland Fusiliers. and promoted to lieutenant on 3 November 1915. Rayner relinquished his acting rank of captain in the Fusiliers on 16 January 1917, and on 22 February was appointed a
flying officer (
observer) in the
Royal Flying Corps with the rank of lieutenant, with seniority from 18 January 1917. He first served as an observer in
No. 52 Squadron RFC, before training as a pilot, receiving
Royal Aero Club Aviator's Certificate No. 5535 on 10 September 1917, Rayner was posted to
No. 89 Squadron RFC, With the Army's Royal Flying Corps and the
Royal Naval Air Service having merged to form the Royal Air Force on 1 April 1918, Rayner then joined No. 60 Squadron RAF, based in France, to fly the
S.E.5a single-seat fighter, being appointed
flight commander of 'A' Flight on 1 August 1918, with the temporary rank of captain. He gained his first aerial victories on the evening on 5 September when he set one German
Fokker D.VII aflame and drove another down out of control over
Avesnes-le-Sec. On the afternoon of 23 October he drove another one down out of control over
Salesches, and on the morning of 25 October he set a D.VII afire and drove another one down out of control over
Berlaimont to become an ace. ==Inter-war career==