Armstrong Whitworth designed the Ensign to seat up to 40 passengers for the airline's European and Asian routes while also carrying airmail.

The Ensign's origins can be traced to 1934 when Imperial Airways expressed a need for a large monoplane airliner powered by four Armstrong Siddeley Tiger radial engines.

[3] During the late 1930s, production of Whitley bombers for the Royal Air Force had priority, which slowed work on the Ensign, and due to a lack of production capacity, assembly was not undertaken at Armstrong Whitworth's main factory in Coventry,[6] but at another member of the Hawker-Siddeley group, Air Service Training in Hamble.

[3] Throughout its development, changes were being requested by Imperial Airways, which delayed progress by up to two years,[7] and the Ensign didn't make its first flight until 24 January 1938.

[8] The two Mark.IIs saw their service entry being further delayed by Imperial Airways and Armstrong Whitworth studying their use as piggyback motherships in the same manner as the Short Mayo Composite's Maia, carrying a smaller long-range parasite aircraft.

[10] During testing, the elevators intermittently jammed while at altitude, which was resolved by modifying the connecting wire runs, to account for the fuselage contracting due to the cold.

A disaster was narrowly averted on 8 March 1938, when all four engines cut out due to an incorrect fuel cock settings, and the aircraft glided to RAF Bicester, making a dead-stick landing.

[11] Flight testing of the prototype found its handling characteristics to be generally acceptable, although Royal Air Force pilots criticised its slow climb rate, which was attributable to the Tiger XI engines that left it underpowered.

[1] The cantilever structure of the wings was built around a single massive internally braced box spar attached directly to both the front and rear ribs.

[15] While most components were designed and manufactured by Armstrong Whitworth, the 16 hydraulic jacks used for the flaps, undercarriage legs, locks and doors were Lockheed units sourced locally.

The Ensign was initially powered by four 800 hp (600 kW) moderately supercharged Armstrong Siddeley Tiger XI[1] 14 cylinder two-row radial engines of 1,996 cu in (32.71 L) displacement[15] mounted ahead of the wing leading edge on a tubular steel framework with flexible rubber mountings and enclosed in Siddeley long chord cowlings.

[18] This improved performance finally allowed the Ensign to be used in hot climates, however production of the engine ended early in the war, and not only did finding spares become difficult, but the rate of climb still generated complaints by the pilots.

[20][1] Armstrong Whitworth claimed that the interior could be reconfigured between each type, including the installation of partition walls and curtains, in 15 minutes.

Three more Ensigns – G-ADSS Egeria, G-ADST Elsinore and G-ADSU Euterpe were completed by Christmas 1938, and were dispatched to Australia with the holiday mail,[4] but all three suffered mechanical problems that prevented them from reaching their destinations.

Following engine trouble in which three engines lost power and began leaking oil during a 3 February 1942 flight that had been following the Takoradi route between Egypt and the British colony in Ghana, Enterprise made a forced belly landing in the Vichy-controlled French West Africa desert about 300 mi (480 km) short of its destination, near Nouakchott, now the capital of Mauritania but at the time a small fishing village.

[28] Deutsche Luft Hansa was invited to evaluate the captured aircraft in early 1943, but they saw it as an obsolete model that had been restored by the French, and its older Cyclone engines were more useful to them in the Douglas DC-3s they were still operating.

[25] During their Certificate of Airworthiness overhauls it was found that the combination of camouflage dope and heat was accelerating the degradation of the fabric surfaces, and thereafter the Ensigns returned to a silver finish.

After the end of the war, in part due to performance and maintenance difficulties with their fabric surfaces and the now obsolete engines, it was decided to withdraw them from service and to return them to the UK.