The Allies of World War II cooperated extensively in the development and manufacture of new and existing technologies to support military operations and intelligence gathering during the Second World War. There are various ways in which the allies cooperated, including the American Lend-Lease scheme and hybrid weapons such as the Sherman Firefly as well as the American-led Manhattan Project. Several technologies invented in Britain proved critical to the military and were widely manufactured by the Allies during the Second World War.
The origin of the cooperation stemmed from a 1940 visit by the Aeronautical Research Committee chairman Henry Tizard that arranged to transfer UK military technology to the US in case of the successful invasion of the UK that Hitler was planning as Operation Sea Lion. Tizard led a British technical mission, known as the Tizard Mission, containing details and examples of British technological developments in fields such as radar, jet propulsion and also the early British research into the atomic bomb. One of the devices brought to the US by the Mission, the resonant cavity magnetron, was later described as "the most valuable cargo ever brought to our shores".
Small arms began to be shared after the fall of France, most of the 'sharing' being one sided as America was not yet directly involved in the conflict and thus all the movement was from the United States to the United Kingdom. In the months following Operation Dynamo, as British manufacturers progressed in building replacements for the materiel lost by the British Army in France, the British government looked overseas for additional sources of equipment to assist in overcoming shortages and prepare for future offensives. The most extreme example of the shortages were found in the quickly improvised Local Defence Volunteers, later renamed the Home Guard, who were forced to train with broom handles and makeshift pikes using lengths of piping and old bayonets until weapons could be supplied.
In addition to those produced in Britain, small arms and ammunition were obtained from Commonwealth countries and also purchased from US manufacturers until they were supplied under lend lease in 1941. The weapons obtained from America for the British army included the Tommy gun, M1911A1 handgun and the Colt .45 revolver. The Home Guard received Browning .30 machine guns in small numbers, M1918 .30 BARs again in limited numbers and P17 .30 Enfield Rifles (the British .303 P14 version also being issued).
Later, the M1919 .30 Machinegun (commonly known as the 30-cal) and the M2HB .50 Machine gun entered service with British Infantry and as anti-aircraft machine guns on British tanks and notably with the SAS on their heavily armed Jeeps. The Browning had been standard fitment on British aircraft since the late 1930s, with the heavy-barrel M2HB Browning .50-cal machine gun still in use by the British military in the 21st century.
Britain supplied small arms to the USSR, and the 9mm Sten Submachine gun was supplied to Soviet partisan troops.
The British made use of many American towed artillery pieces during the war, such as the M2 105 mm howitzers, M1A1 75mm Pack Howitzers, 155 mm guns (Long Toms). These weapons were supplied under Lend-Lease or bought outright. Tank/tank destroyer guns used by the British included the 37 mm M5/M6 Gun (General Stuart and General Grant/Lee tanks), 75mm M2 Gun (General Grant/Lee), 75 mm M3 Gun (General Grant/Lee and General Sherman), 76 mm Gun M1 (General Sherman) and 3" Gun M7 (3in SP M10).
The Americans in turn used a British artillery piece, the Ordnance QF 6-pounder 7cwt anti-tank gun. The US realized at the start of the war that their own 37 mm Gun M3 would soon be obsolete and thus they produced a license built version of the QF 6-pounder under the designation 57 mm Gun M1.
Both 76 mm and 75 mm guns were mounted on tanks sent to the Soviets by the US, while the British tanks sent were armed with both the Ordnance QF 2-pounder and the Ordnance QF 6-pounder.
Another technology taken to the US, by Tizard, for further development and mass production, was the (radio-frequency) proximity fuse. It was five times as effective as contact or timed fuzes and was devastating in naval use against Japanese aircraft and so effective against German ground troops that General George S. Patton said it "won the Battle of the Bulge for us."
The Medium Tank M4 was used in all theatres of the Second World War. It had a versatile reliable design and was easy to produce, thus huge numbers were made and provided to both Britain and the USSR by the United States under Lend-Lease. Despite official opinions, the Medium Tank M4 was well liked by some Soviet tankers, while others called it the best tank for peacetime service. When Britain received the tank, it was given the designation Sherman, which gave rise to the name Sherman tank and the UK naming its US-built tanks after American Civil War generals. Both the British and the Soviets re-armed their M4s with their own tank guns. The Soviets re-armed a small number with the standard 76 mm F-34 tank gun but so much 75 mm ammunition was supplied by the US that the conversions were not widespread. The British conversion was better known as the Sherman Firefly, which had the 3-inch (76.2mm) calibre Ordnance QF 17-pounder anti-tank gun mounted in the standard turret with a new mantlet and the radio moved to an armoured box welded to the turret bustle. The combination of British and American weaponry proved desirable, although despite the United States building a few 17-pounder Fireflies from new, it never went into mass production and did not see action. The US had its own reasonably effective 76 mm calibre long-barrel gun for the Sherman, which was much-used later in the war. The Firefly thus remained a British variant of the Sherman. The M10 Tank Destroyer was also up-gunned with the 17-pounder, creating the M10C tank destroyer. This was used in accordance with British tactical doctrine for tank destroyers, in that they were considered self-propelled anti-tank guns rather than aggressive 'tank hunters'. Used in this fashion, it proved an effective weapon.
The British also used the Sherman hull for two other Sherman variants known as the Crab, a mine flailing tank, and the DD Sherman, the 'DD' standing for Duplex Drive. The DD was an amphibious tank. A flotation screen gave buoyancy and two propellers powered by the tank's engine gave propulsion in the water. On reaching land the screens could be dropped and the tank could fight in the normal manner. The DD, another key example of combining technologies, was used by both British and American forces during Operation Overlord. The DD had impressed US General Dwight D. Eisenhower during demonstrations and was readily accepted by the Americans. The Americans did not accept the Sherman Crab, which could have assisted combat engineers with clearing mines under fire, protected by armour. Armoured recovery vehicles (ARVs) were also converted from Shermans by the British as well as the specialist BARV (Beach Armoured Recovery Vehicle) designed to push-off landing craft and salvage vehicles which would otherwise have been lost.
The British supplied tanks to the USSR in the form of the Matilda, Valentine and Churchill infantry tanks. Soviet tank soldiers liked the Valentine for its reliability, cross country performance and low silhouette. The Soviet's opinion of the Matilda and Churchill was less favourable as a result of their weak 40-mm guns (without HE shells) and inability to operate in harsh rasputitsa, winter and offroad conditions.
Deliveries of M3 Half-tracks from the US to the Soviet Union were a significant benefit to mechanized Red Army units. Soviet industry produced few armoured personnel carriers, so Lend-Lease American vehicles were in great demand for fast movement of troops in front-line conditions. While M3s had only limited protection, common trucks had no protection at all. In addition, a large part of the Red Army truck fleet was American Studebakers, which were highly regarded by Soviet drivers. After the war, Soviet designers paid a lot of attention to create their own 6x6 army truck and the Studebaker was the template for this development.
Britain supplied Hawker Hurricanes to the Soviet Union early in the Great Patriotic War to help equip the Soviet Air Force against the then technologically superior Luftwaffe. British RAF engineer Frank Whittle travelled to the US in 1942 to help General Electric start jet engine production.
The American P-51 Mustang was originally designed to a British specification for use by the Royal Air Force and entered service with them in 1942, and later versions were built with a Rolls-Royce Merlin aero-engine. This engine was being produced in the United States by Packard as the Packard Merlin. In addition to the British making use of American planes the US also made use of some Supermarine Spitfires based in the UK and Mediterranean, as well as using Bristol Beaufighter night fighters in the Mediterranean, and de Havilland Mosquitoes based in the UK.
The United States supplied several aircraft types to both the Royal Navy and RAF - all three of the U.S. Navy's primary fighters during the war years, the Wildcat, Corsair (with the RN assisting the Americans with preparing the Corsair for U.S. naval carrier service by 1944), and Hellcat also served with the RN's Fleet Air Arm, with the Royal Air Force using a wide range of USAAF types. A wide range of American aircraft designs also went to the Soviet Union's VVS air arm through Lend-Lease, primarily fighters like the P-39 and P-63 used for aerial combat, along with attack and medium bombers like the A-20 and the B-25 being among the more prominent types, both bombers being well suited to the type of lower-altitude strike missions the Soviets had as a top priority.
In 1942, and with the threat of invasion by Germany still apparent, the United Kingdom dispatched around 20 British scientists and technical staff to America, along with their work, which had been carried out under the codename Tube Alloys, to prevent the potential for vital information falling into enemy hands. The scientists joined the Manhattan Project at Los Alamos, New Mexico, where their work on uranium enrichment was instrumental in jump-starting the project. This collaboration eventually led to the Mutual Defence Agreement between the two nations, whereby American nuclear weapons technology was adapted for British use.
Considerable information was transmitted from the UK to the US during and after WWII relating to code-breaking methods, the codes themselves, cryptoanalyst visits, mechanical and digital devices for speeding code-breaking, etc. When the Atlantic convoys of war material from the US to the UK came under serious threat from U-boats, considerable encouragement and practical help was given by the US to accelerate the development of code-breaking machines. Subsequent co-operation led to significant success in Australia and the far East for breaking encrypted Japanese messages.