Barnwell was one of the first designers in Britain to adopt the new technology of the stressed-skin monoplane, starting design of a high-speed six-seater in 1933. This developed into the 'Britain First', predecessor of the Blenheim bomber.

Like his brother before him, Frank Barnwell suffered an untimely death, killed as he was in a flying accident in 1938 whilst testing a machine built for his own use. Frank Barnwell loved to fly but unlike his brother Harold, he was not a great pilot and Bristol finally stopped him flying the company's aircraft. Frank, therefore designed and built a motorcycle engined ultra-light for his own use and on its second flight at Whitchurch, Bristol, he crashed and was killed outright. The world had lost, still with so much to give, one of the most innovative aircraft designers of all time. In World War Two, Frank's three sons joined up for active service, and tragically, they also lost their lives, two of them in Blenheims.

Barnwell's last design, in a career which had helped aviation develop from 'stick and string' to the powerful all-metal monoplane, was the Beaufort torpedo bomber which first flew 11 weeks after his death. C G Grey, editor of 'The Aeroplane', called Frank Barnwell "one of the most charming people one could meet", adding that in 27 years he had never seen him angry. He was also, Grey said, "beyond question, one of the best aeroplane designers ' in the world."