Frost had then become heavily involved in one of the most important new developments at the time- swept wings and a tailless configuration on a jet fighter. Designer and company founder, Sir Geoffrey de Havilland, had already begun the D.H.106 Comet development process and was considering that radical configuration for the world's first jet airliner. As Project Engineer on the D.H.108, with only a team of 8-10 draughtsmen and engineers, Frost created a remarkable aircraft by marrying the front fuselage of the Vampire to a swept wing and short stubby vertical tail to make the first British swept wing jet soon to be unofficially known as the "Swallow". The elegant and sleek experimental D.H.108 was also to serve as a test "mule" to investigate stability and control problems for the new Comet airliner.

The D.H.108 first flew on May 15, 1946, a mere 8 months after Frost had a go-ahead on the project. Company test pilot and son of the builder, Geoffrey de Havilland Jr., flew the first of three aircraft and found it extremely fast- fast enough to try for a world speed record. On April 12, 1948, a D.H.108 did set a world's speed record at 973.65 km/h (605 mph) and later on became the first jet aircraft to exceed the speed of sound. The first D.H.108, TG-283, was alleged to have suddenly jumped from Mach .98 to Mach 1.05 while being test-flown by John Derry on September 9, 1948. Derry's passage through the sound barrier, which he stated occurred during an uncontrolled dive, remains unofficial since a recording camera was inoperative at the time. The seminal event in its short lifespan had already occurred, however. On September 27, 1946, while practicing for an upcoming run at a new speed record, Geoffrey de Havilland Jr. died when his D.H.108 broke up in the air at or near the speed of sound.

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