[Shadowcougar]

The A-36 was the the P-51A with .50 cal mg and dive brakes.
The P-51 used with 4x 20 mm cannons was the Mustang I and was used as a low level attack craft. They used a common airframe and engine that was only good at lower levels. The Mustang became a supreme fighter when then added the Merlin engine

http://home.att.net/~jbaugher1/p51_6.html
http://home.att.net/~jbaugher1/p51_7.html
http://en.wikipedia.org/wiki/P-51_Mustang