In the mid-1930s, the British Air Ministry released Specification 36/35 calling for a transport / mailplane aircraft capable of a transatlantic crossing. The de Havilland concern returned with its DH.91 design drawn up by aircraft engineer A.E. Hagg (1888-1985). The DH.91 lived a relatively short service life as its production and exposure was ultimately limited by the arrival of World War 2 (1939-1945). As such, totals included just seven useable airframes - including two flyable prototypes.
The aircraft used a largely conventional arrangement which included a stepped cockpit overlooking the nose, low-set monoplane wings, and a twin-finned tail unit. The fuselage was relatively deep and dotted by rectangular windows while also being highly streamlined for aerodynamic efficiency. Each wing mainplane carried a pair of engine nacelles and these were equally-streamlined, hugging their mechanical components as close as possible, to further the aerodynamic qualities of the aircraft. This sort of component-hugging benefit was made possible by an all-new cooling system implemented into the design of each unit. Construction also included a unique sandwiched (plywood-balsa) wood arrangement used across the fuselage - a construction technique later pressed onto the war-winning de Havilland "Mosquito" fighter-bomber of World War 2 fame (this aircraft appears in detail elsewhere on this site).
As completed, the DH.91 would field a crew of four to include two pilots, a radioman, and a flight steward. Up to twenty-two seated passengers could be carried if the fuselage was arranged in such a way to accommodate them. Overall length reached 71.5 feet and the wingspan was 105 feet with the height measuring 22.2 feet. Empty weight was 21,230lb against an MTOW of about 30,000lb. Power was served from 4 x de Havilland "Gipsy Twelve" 12-cylinder, air-cooled, inverted-Vee piston engines outputting 525 horsepower each.
Performance went on to include a maximum speed of 225 miles per hour, a cruising speed near 210 miles per hour, a range out to 1,040 miles, and a service ceiling up to 18,000 feet. Rate-of-climb was 700 feet-per-minute.
A first-flight of the DH.91 in prototype form occurred on May 20th, 1937 and a second flyable form was completed and flown thereafter. The second example suffered a complete fracture under load during testing which forced some reinforcement of the airframe. Imperial Airways became the launch customer of the series and took into inventory both of the prototypes and a further five production models (totaling seven airframes). The first example received during October 1938 was converted to seat twenty-two passengers in some comfort while the two prototypes were received in their original mail-carrying forms. The fleet was used to connect locations within the UK to parts of France, Belgium, and Switzerland during the early-going of operational service.
As was common during the period, this small group of aircraft were named as naval ships would be. They fleet constituted "Faraday", "Franklin", "Frobisher", "Falcon", "Fortuna", "Fingal", and "Fiona".
The arrival of World War 2 on September 1st, 1939, changed how Europe did business in the air. The Royal Air Force (RAF) was in dire need of any and all capable aircraft and studied the DH.91 for its range qualities. As such, a pair of the mail carrier forms (Faraday and Franklin) were taken into military service through Squadron No.271 during September of 1940 and these flew a route between the UK and Iceland. However, both suffered from accidents during landing actions while on approach at Reykjavik which ended their flying days in the war heading into 1942. The remaining five passenger haulers served with BOAC (born from the reorganization of Imperial Airways) and these continued in service by connecting Bristol to Ireland and to Portugal though one (Frobisher) was destroyed during a German air raid and another (Fingal) in a landing accident in 1940. Another (Fortuna) example crashed after its now-weakened wood structure gave out in 1943 near Shannon Airport in Ireland.
The remaining two aircraft - Falcon and Fiona - were voluntarily destroyed in September of 1943 rather than continuing to press these airframes any further. Such ended the short-lived reign of the de Havilland DH.91.