The North American P-51 Mustang was designed for the RAF early in WW2. It entered combat service in 1942.
Fitting Mustangs with the Rolls Royce Merlin engine made a good fighter into a great one, and with the addition of drop-tanks the allies had the long-range escort fighter they needed so badly.
The Mustang 'B' was armed with 4 x 12.7mm machine guns, and could also carry drop-tanks or bombs.
This example is a P-51B of the 328th Fighter Squadron USAAF circa 1944.