Frank joined Bristol Aircraft in 1911, enlisting in the Royal Flying Corp after war broke out. Aircraft and pilot losses were high and the British aircraft were clearly inferior to the German ones. In 1915 he was sent back to Bristol as Chief Engineer.
Aircraft design involves balancing a number of potentially conflicting design requirements whose treatment and optimisation was not understood at the time. Barnwell laid out the seven consecutive pivot points of aircraft design in a paper read to Glasgow University Engineering Society in 1915, (published as a book in 1916). The resulting Bristol Fighter became a legend almost as soon as it arrived on the Western Front and 2600 were built.
His importance is laid out in an article by the Curator of Aerodynamics at the US Smithsonian Institution: "Frank Barnwell established the intellectual thought process that underlies the methodology of conceptual airplane design to this day."