In 1934, there was a debate within the RAF about whether its next generation of fighters, the Hawker Hurricane and the Supermarine Spitfire, should have four or eight 0.303 inch Colt Browning machine guns. These guns had the same calibre of ammunition as the British Army’s Lee Enfield rifles.
This is well known but the role of Hazel Hill, a 13 year old schoolgirl, in the decision to adopt eight guns has only recently come to light thank to a BBC News Channel documentary called The Schoolgirl Who Helped to Win a War. It is based on research carried out by her grand-daughter Felicity Baker, a journalist. It is available to UK viewers on the iPlayer at the link below.
There were two alternative guns that might have armed the new fighters. The 20mm Hispano cannon, used by the French, was ‘new and temperamental’
Stephen Bungay, author of The Most Dangerous Enemy: A History of the Battle of Britain (London: Aurum Press, 2009), stated on the BBC documentary that the 20mm cannon and the 0.5 inch machine gun were rejected on the grounds of weight.
In 1934, RAF fighter squadrons were equipped with Bristol Bulldogs and Hawker Furies and were soon to receive Gloster Gauntlets, fabric covered biplanes with open cockpits, fixed undercarriages and an armament of two 0.303 inch machine guns. The next British fighter, the Gloster Gladiator, had an enclosed cockpit and four 0.303 inch machine guns, but was still a fixed undercarriage biplane. Most air forces in the world were then equipped with similar fighters. The best fighter in the world was probably the Polish PZL P.11, an all metal gull winged monoplane, but it still had an open cockpit, a fixed undercarriage and only two machine guns, although later models carried four.
The world’s leading air forces were about to introduce monoplanes with retractable undercarriages, enclosed cockpits, greater speed, higher altitude ceilings and heavier armament. The question for the British was how many guns their new fighters carry.
Squadron Leader Ralph Sorley, head of the Air Ministry’s Operational Requirements Section, thought that eight guns were required to to inflict enough damage to shoot down an enemy aircraft in the two seconds that a fighter pilot was expected to be able to keep it in his sights. Air Marshall Edgar Ludlow-Hewitt, the Deputy Chief of the Air Staff, agreed.
Others, however, were not convinced, including Air Marshall Robert Brooke-Popham, Commander-in-Chief of the Air Defence of Great Britain and thus the commander of the fighter squadrons that would receive the new aircraft. He thought that eight guns were too many and that four were enough, arguing that eight would create ‘a lot of leading edge resistance.’
Captain Frederick Hill, the Air Ministry’s Senior Technical Officer of Ballistics, was given the job of calculating the number of guns needed by a modern fighter. He came from a working class background but received a BSc in Chemistry from London University in 1909 and subsequently worked as a teacher. He served in the Royal Naval Air Service and the Royal Air Force during the First World War, working on the technical aspects of aerial guns and gunsights. He continued with this work after the war as a civilian employee of the Air Ministry, although he almost lost his job during defence cuts in 1922.
Hill was given the task of working out how many guns the new fighters needed. As he was working to a tight timetable, he took the relevant documents and a calculating machine home and worked on his kitchen table, helped by his 13 year old daughter Hazel, a maths prodigy. Their calculations showed that that with machines guns that fired 1,000 rounds per minute, a fighter would need eight guns to cause enough damage to destroy an enemy aircraft in the two seconds that it was likely to have the enemy in its sights.
The next day Hill presented his conclusions to an Air Ministry committee, which accepted that his recommendation that the new fighters needed to have eight machine guns. According to the Times article linked below, he told only his immediate superior that Hazel had helped him with the calculations. The BBC documentary said that his superior mentioned this in his memoirs. Neither source names him but he was presumably C. H. Keith, whose book I Hold My Aim is referred to in the Wikipedia entry on Hill.
The BBC documentary interviewed Squadron Leader Allan Scott, a 98 year old who flew both Hurricanes and Spitfires in the Second World War. In his opinion, they could not have shot down enough enemy aircraft to win the Battle of Britain had they been armed with only four machine guns.
Even eight 0.303 machine guns was soon regarded as being too weak an armament. During the Battle of Britain, Spitfires and Hurricanes fought against German Messerschmitt Me109E fighters that carried two 20mm cannon and two 7.92mm machine guns. In 1941, the Spitfire VB, armed with two 20mm cannon and four 0.303 inch machine guns, and the Hurricane IIC with four 20mm can non entered service.
Captain Hill died in 1959. Hazel followed a career in medicine rather than Maths, serving in the Royal Army Medical Corps during the Second World War. She died 10 years ago. Her four sons appeared in the BBC documentary and her family still owns the table on which the calculations were made.
 E. B. Morgan, E. Shacklady, Spitfire: The History, Rev. ed. ed. (Stamford: Key Books, 1987), p. 18.
 A. Price, The Spitfire Story (London: Arms and Armour, 1982), p. 21.
 Quoted in Morgan, Shacklady, Spitfire: The History, p. 15.