The need of a replacement for the D.H.10s used on the RAF's air mail service between Cairo and Baghdad, coupled with an agreement reached in 1925 for Imperial Airways to take over the service, led to a requirement being issued which was met by the de Havilland D.H.66 Hercules, a three-engined biplane with a 155 cu ft (4.39 m³) baggage compartment, space for seven passengers and 465 cu ft (13.17 m³) of mail and a three man crew.
The prototype flew on 30th September 1926 following receipt of an order for five aircraft from Imperial Airways. Such was the speed and comparative simplicity of procedures in those days that the prototype carried out acceptance flights, took part in some crew training and was delivered to Cairo by mid-December. An inaugural flight between Croydon and India left the UK on 27th December and arrived in Delhi on 8th January 1927.
The fifth aircraft was delivered to Cairo in March 1927. The performance of these aircraft impressed West Australia Airways, then using D.H.50s. Four examples of the Hercules were ordered, the first flying in March 1929, and the type entered service with WAA on the Perth-Adelaide route on 2nd June. By then Imperial had ordered a sixth aircraft and its seventh and final aircraft followed in February 1930.
Imperial's sixth Hercules had an enclosed pilot's cabin, a modification which later became standard on the remaining aircraft. The airline's need for these last two Hercules followed the loss of three in crashes between September 1929 and April 1931, but only the first caused fatalities. Aircraft shortage led to the purchase by Imperial of two WAA Hercules in 1930-1; one of these crashed in Southern Rhodesia in November 1935 and Imperial eventually withdrew its last aircraft from service in December 1935, having sold three to the South African Air Force. Their eventual history is not known, but the longest surviving Hercules was probably one of the two former WAA aircraft, being used in New Guinea between Lae and Wau and destroyed by enemy action in 1942.