De Havilland DH9s flying in formation. From a copy negative made in 1923. Designed in 1916 by Airco, it first saw action with the Royal Flying Corps over the Western Front in 1917, but the aircraft went out of service with the RAF in 1920. Large numbers were subsequently exported for use by Commonwealth Air Forces or were adapted for use by civilian air transport companies. Some adaptations remained in use until 1936. Aerofilms Collection (see Links).