Soon after the appearance of the British D.H. 9 long-range bomber was made the deficiencies of the aircraft became evident. When the Americans announced the development of the Liberty engine the British decided to modify the D.H. 9 to incorporate the new engine. This necessitated the incorporation of a strengthened fuselage to handle the heavier engine plus new main planes in increased span and chord. Also, a new radiator design was incorporated. The new design was designated as the D.H. 9A. The first modified aircraft was ready for flight in early 1918, but the Liberty engine was not. Therefore, the first flight, which occurred in March of 1918, flew with the Rolls-Royce Eagle VIII engine. The Liberty engined flight did not occur until 19 April 1918. The ordnance carried by the aircraft consisted of one fixed 0.303 in Vickers machine gun, synchronized by a Constantinsco CC Gear for the pilot and one 0.303 in Vickers machine gun on a Scarff ring for the Observer. The bomb load was six 60 lb. bombs.

About this time the First Marine Aviation Force of the Navy's Northern Bombing Group was short of aircraft and was borrowing or using RAF machines on their missions. The situation was that the Marines had a surplus of Liberty engines but no aircraft while the RAF had a surplus of aircraft, but no engines. Therefore, an agreement was reached whereby three Liberty engines would be exchanged for one D.H. 9A fuselage. By the end of the war some 20 D.H. 9A's were with the Marines; out of a total of some 57 D.H. 9A's transferred by the British to the Navy