Every evening, once the dinner dishes had been cleared away, Hazel Hill would sit at the kitchen table prodding numbers into her father’s calculating machine. It should never have left his office at the Air Ministry but Fred had a mountain of data to process and his 13-year-old daughter was a brilliant mathematician. Battle of Britain pilots, locked in dogfights with powerful German fighters, would soon have cause to be grateful for the work carried out in that Highgate kitchen. The Spitfire Kids.

Fred Hill

Fred Hill was a gunnery expert.  In the First World War he worked at the Isle of Grain seaplane base, developing gunsights for the Navy and the Royal Flying Corps.  By 1930 there were very few people in the world who knew more than Fred about the complex science of hitting a moving aircraft with a stream of bullets.

In 1931 Hill helped organise a first in British aviation- the Martlesham firing trials.  An aircraft towed a target attacked by a fighter armed with the standard twin Vickers machine guns.  The trials quickly revealed the size of the task facing the designers of the new generation of fighter aircraft.  RAF aerial combat strategy was to open fire from the maximum range of 900 metres, much as a battleship might do.  The trials showed no hits at all at this range.  Hill became convinced that the only chance of a fighter taking down a fast enemy bomber was to get close and fire as many bullets as possible as quickly as possible.

The pace of engine development in Germany, Italy, the US and Great Britain meant that the next generation of bombers would be flying as speeds close to 400mph. Fighter pilots defending Britain would have just moments to make their attack count. They would need much more firepower.

Fred Hill testing a gun in 1917

Hill needed to prove his point before the Royal Air Force’s new fighters – the Hurricane and the Spitfire – entered production. He was already busy developing a new deflector gunsight for the fighters – any extra work would have to be done in his own time.   His solution was simple, daring and almost certainly in breach of the Official Secrets Act. He took the figures home and set his teenage daughter to work.

Labelled as naughty at school, Hazel Hill struggled with words but handled numbers effortlessly. At their terrace house in North London, Fred and Hazel fed figures into the hand-cranked calculator. Its innovative accumulator memory made it possible to store data as they went along and Fred and Hazel gradually plotted out the relationship between aircraft speed, firing range and density of fire.  The results were startling.

The .303 Browning machine gun

Hazel and Fred’s calculations proved that a fighter would have to get close, very close, to have any impact.  It would have to fire just 230 metres from the target.  At that distance, chasing a fast, powerful bomber, the pilot would have the enemy in his sights for two seconds.  With four of the latest Browning machine guns that would put around 130 bullets in the bomber, not enough to bring it down. Eight guns would put 260 bullets into the enemy.  That should do the job.

Squadron Leader Ralph Sorley of the Air Ministry’s Operational Requirements Branch was convinced by Fred’s case and in April 1935 visited the Supermarine factory in Southampton where R.J. Mitchell was putting the finishing touches to his Spitfire prototype. Sorley asked if this four-gun aircraft could be adapted and found the short-tempered Chief Designer unusually amenable.

Hazel and Fred’s long nights at the kitchen table had not been in vain.  The new specification arrived just in time for Supermarine and Hawker to adapt their plans.  The Spitfires and Hurricanes that fought the Battle of Britain would face the heavily-armed Messerschmitt 109s with eight machine guns, not four.

Hazel would have her own reasons to thank the Spitfire.  At the outbreak of war she was studying to be a nurse at Marischal College in Aberdeen.  It should have been a safer spot than North London but in early 1940 the Luftwaffe was engaged in small-scale raids on Britain’s east coast.  Hazel was walking home from lectures along Union Street, the city’s main drag, when a bomber roared up behind her and began strafing the shoppers.  She threw herself to the granite pavement, covered her ears and looked up to see a very welcome sight, “A nice young man in a Spitfire was chasing him away.”

Alasdair Cross is the author of The Spitfire Kids and producer of the BBC podcast, Spitfire: The People’s Plane

The Spitfire Kids by Alasdair Cross is published by Headline on 13th May (Hardback, £20.00)