The P-51 Mustang and P-47 Thunderbolt were the finest American fighters of World War 2, and both saw service with the RAF in substantial numbers. The RAF began flying the Mustang in 1944, using it to fly bomber escort missions, and deploying to support the ground campaigns in Italy and the Balkans. It was also flown by a number of Polish units in the RAF. The P-47 Thunderbolt was the best fighter available in CBI theatre, where it was flown by a number of aces against the Japanese, mainly in a ground support role. Although these two fighter types are most associated with the Americans, they performed an important role serving in the RAF.