Royal Naval Air Service
Middleton initially served in the Royal Naval Air Service, as a member of No. 5 (Naval) Squadron of the Royal Naval Air Service, flying the Airco DH.4 two-seater day bomber. He gained his first aerial victory on 9 March 1918, driving down an Albatros D.V out of control over Mont-d'Origny, while being piloted by Lieutenant G. E. Siedle. Soon after, on 1 April, the Royal Naval Air Service and the Army's Royal Flying Corps were merged to form the Royal Air Force, and Middleton's unit became No. 205 Squadron RAF.