The P-51 Mustang was an American fighter made by North American Aviation during World War II. Later versions escorted American bombers over Europe and Japan. There were 5 versions of the Mustang built, called the P-51A, P-51B, P-51C, P-51D, and the P-51H. To provide more power at high altitude, the B, C, D, and H versions had a Packard-built version of the Rolls-Royce Merlin engine instead of the Allison engine used in the P-51A. The Packard Merlin engine featured a more advanced supercharger, allowing much better performance above 15,000 feet when compared to the Allison. The P-51D version had a "bubble" canopy, or one unobstructed by metal supports. After World War II, it was used in the Korean War as a dive bomber and was used by many Central American air forces until the 1970s. Today, P-51 Mustangs are often used in air races.