The Leigh Light was developed to solve a problem with anti-submarine radar during the Second World War. By 1941 the British had developed radar systems capable of detecting a surfaced U-boat, but interference from the surface of the sea meant that the radar signal would be lost during the final attack run.
The solution to this problem was to fit a bright light to the attacking aircraft. Two competing designs were developed. The first, developed by Group Captain Helmore, involved fitting a wide-angle light in the nose of a Wellington bomber. The second, developed by Squadron Leader Humphrey de Verd Leigh, used a controllable spotlight suspended below the belly of the aircraft. Leigh took advantage of the hole in the base of the Wellington left by the removal of the ventral turret used on early Wellingtons. Fraser-Nash, who had developed the original turret, produced a modified version that could carry the spotlight, a 60 cm naval searchlight. Power was originally provided by a Ford V8 engine powering a generator, but that was soon abandoned in favour of rechargeable batteries.
After tests in May 1941, the Leigh Light was selected for use. The nose-mounted Helmore light required the entire aircraft to be pointing at the U-boat, causing problems on the approach, while the Leigh Light was fully controllable. Fitting the light to a Wellington required some major changes. The forward turret had to be removed to make space for the Leigh Light controls. To provide some forward firepower two manually controlled .303in machine guns were placed in the forward canopy. The first unit to receive the Leigh Light was 1417 Flight, who received lights for their Wellington VIIIs in January 1942. In April 1942, the flight became No. 172 Squadron.
The first attack to use the Leigh Light came on 4 June 1942. The Italian submarine Luigi Torelli was crossing the Bay of Biscay on its way from La Pallice to the West Indies. At the same time a Wellington Mk VIII of No. 172 Squadron, piloted by Squadron Leader J. H. Greswell, was patrolling in the area. At just after 1.27 am the submarine was detected. Greswell made his approach, and turned on the light. It operated exactly as hoped. The Italian submarine was identified and its location confirmed. The submarine commander, unaware of the new weapon, remained on the surface while Greswell made his attack. Four depth charges straddled the submarine, which was badly damaged, although not sunk. The submarine was forced to run for safety in neutral Spain, reaching Santander by 8 June (despite two more attacks by R.A.F. Sunderlands). The first confirmed kill came a month later, on 5 July, when a Wellington piloted by Pilot/Officer Howell, one of many Americans who had joined the RAF, sank U-502 in the Bay of Biscay.
Not every Coastal Command aircraft could carry the Leigh Light. An attempt was made to fit it to the Halifax bomber in 1944, but the edge of the bomb bay blocked the light. No attempt was made to fit the Leigh Light to the Sunderland. It was used with the Liberator, which had much longer range than the Wellington. The Leigh Light remained in use throughout the war – at the end of 1944 Coastal Command had 119 Leigh Light equipped Wellingtons. Although the Leigh Light was not responsible for a large number of confirmed U-boat kills, it did force the U-boat force to abandon the surface of the Bay of Biscay at night and generally made life much harder for the U-boat crews. Prior to the appearance of the Leigh Light, the U-boats had been safe on the surface at night, using the time to refresh their air and recharge their batteries.