The P-51 was used in WWII on the Allied side. Mainly as an bomber escort over germany, but also in the pacific war. Because of these escorts they were designed to travel very far. The P-51 became the USAF's flagship fighter in WWII.
Most of the P-51's wartime escorts were over germany defending the B-17s from attack. It also saw combat in the pacific war against Japan. The P-51 was used in the Korean war as well. It was one of the last propellar fighters to be used being replaced by jets in later years.
Although more expensive to build the P-51 was very durable and though to take down. The Japanese in WWII had trouble matching this and instead built their planes to be built quickly in order to swarm the enemy. This tatic worked as long as the pilots didn't panic.
World War Two
At a conferance during WWII the allies developed a plan to bomb germany around the clock. The RAF would bomb by night and the USAF by day. THe USAF needed fighters to defend their bombers against enemy fighters. The P-51 was their answer.