[2] Between 1929 and 1934, altitude records established by rival aircraft included those set by a Junkers W.34, a Vickers Vespa and a Caproni Ca.113 biplane, as well as the first flight over Everest by a pair of Westland Wallaces in 1933.

[2] In November 1933, having observed British Air Ministry interest following the success of the Everest flight, aeronautical engineer Frank Barnwell proposed a purpose-built high-altitude research aircraft.

This proposal, designated the Type 138, was a large single-engine, single-seat monoplane, equipped with a retractable undercarriage and a supercharged Pegasus radial engine.

Aviation publication 'Flight observed of the aircraft that: "except for its size, reminds one very much of the little Bristol Brownie.... the machine is the largest single-seater aeroplane ever built".

[11] The pilot was seated in a spacious cockpit, which was heated by air directed from the oil coolers set within the wings, which could be adjusted.

[14][2] It with a plywood skin glued to the mahogany longerons and struts that formed the internal structure, which was faired throughout to reduce drag.

[8] It was completed with closed-circuit breathing apparatus with oxygen being delivered via a small injector jet to provide air circulation.

Exhaled air travelled via an external tube to a canister containing carbon dioxide-absorbing chemicals to restore it prior to it returning to the pilot again.

[14] A specially-developed fuel, known as S.A.F.4, was used for the altitude record flight, derived from standard grade Shell Ethyl aviation gasoline.

Swain ran low on oxygen on the two-hour flight and had to break the window of his pressure helmet after descending to a safe height.

[15][9] After this flight, further development work resulted in a number of small modifications to the aircraft, the typical objective of these being weight savings and improving the performance of the supercharger.

[2] This was to be a two-seater powered by a Rolls-Royce Kestrel S engine, fitted with a similar two-stage supercharger installation, enabling it to generate 500 hp (370 kW).