Aircraft pioneer. Born in Lewisham (Kent), the son of Richard Barnwell who was to become a director of Fairfields, the Clyde shipbuilder, and the elder brother of Frank Barnwell (1880 - 1938). Barnwell was brought up at Elcho House in Balfron and educated Fettes College in Edinburgh. Harold and Frank visited the Wright Brothers in America and returned to their new family home in Bridge of Allan where they were able to fund their aeroplane designs through a new business, the Grampian Motor and Engineering Company setup at nearby Causewayhead. Harold was to successfully fly a biplane of their design in 1909, the first powered flight in Scotland. Next the brothers built a monoplane which flew over such a distance that Barnwell won a prize of £50 offered by the Scottish Aeronautical Society.
Barnwell went on to work for Vickers, becoming their Chief Test Pilot at their factory in Brooklands (Surrey) in 1912. He helped design the Vickers FB5 Gun Bus, mainstay of the Royal Flying Corps in the early years of World War I, and the pioneering Vickers ES1 or Barnwell Bullet.
He was killed when a prototype of the Vickers FB26 Vampire he was testing crashed at the Joyce Green Aerodrome, near Dartford in Kent.