During WWII military aviation increased dramatically. Planes played a huge role during the war, some battles were even fought exclusively in the air. The United States manufactured around 300,000 aircraft during the war and as the war continued planes became increasingly more advanced and powerful.

Aero engines were designed with altitude capability and power in mind. Fighter and bomber planes by the late 1930s had 2,000 horsepower engines that were radial air-cooled and inline liquid cooled. Fighter planes, which needed more power, received supercharged liquid-cooled inline engines such as the Rolls-Royce, Allision, or Daimler Benz, all of which are on the top 10 list of engines produced in the greatest quantity during the war. 

During WWII, thousands of Rolls Royce Merlin engines powered fighting air crafts. First manufactured in Great Britain, the engines originally powered automobiles. It wasn't until WWI that they began to be produced for aircraft. During the Second World War, the engines only grew in importance as they powered many war planes including the Supermarine Spitfire, DeHavilland Mosquito, and Avro Lancaster. 

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Today Rolls Royce engines are recognized as one of the most successful engines of the WWII era and the worlds second-largest maker of aircraft engines.

Daimler Benz Engines were first manufactured in Germany in the 1940s. It was a liquid-cooled 12-cylinder inverted V12 Engine and powered several WWII planes, including the Do 217 N&M, Do 335, He 219, and Me 410. 

The Allison V12 Engine was the only American liquid-cooled engine to see service in World War II. It was one of the most important American aero engines that powered most of the US Army planes during WWII. Over 70,000 engines were produced from 1931 to 1948. The engine was found in several important fighter planes including the Lockheed P-38, Bell P-39 and P-63, Curtis P-40.