During the later stages of WWII, Allied bombers ventured deep into Germany but faced significant losses due to the limited range of their fighter escorts. German fighters would remain out of reach of the Allied escorts and then strike at the unprotected bombers. The situation changed when the British-modified P-51 Mustang, originally equipped with an Allison engine, was upgraded with a British Merlin engine. This modification provided the P-51 with the high performance it had previously lacked. With the addition of drop tanks, the P-51 gained the range needed to escort bombers all the way to Berlin and back.