Armament
Armament varied throughout the life of the Mustang. Initial versions were fitted with 6 x 12.7mm machine guns - two in the nose and four in the wings. A battery of 4 x 20mm cannons, which made it ideal in the ground attack role, or the lighter armament load of 4 x 12.7mm machine guns (two to a wing) for tactical reconnaissance were also alternatives. Eventually, the legendary D-models would introduce the potent array of 6 x 12.7mm machine guns (three to a wing) with a simplified feed mechanism to cut down on weapon jamming. Underwing bomb racks and rocket pylons increased potency of the platform as well. These could be deleted in favor of fuel drop tanks for improved range on those long bomber escort sorties.
http://www.militaryfactory.com/aircraft/detail.asp?aircraft_id=77pic of A36 with 4 x 20mm's
http://www.theospark.net/2009/01/p-51-mustang-california-1942.html model by model of P-51 / A-36
Mustang Mk IA: Supplied to the Royal Air Force (RAF).2,4 Had cannons for armament.4 Originally called Apache by the United States Army Air Force.4
A-36A: Ground attack.2,3 Had wing bomb shackles.2,3 Had four 20 mm cannons in the wings.3
http://www.wwiivehicles.com/usa/aircraft/fighter/north-american-p-51-mustang.aspA second British contract called for 300 more (NA-83) Mustang Mk.I fighters. In September 1940, 150 aircraft, designated NA-91 by North American, were ordered under the Lend/Lease program. These were designated by the USAAF as P-51 and initially named Apache, although this was soon dropped and the RAF name, Mustang, adopted instead. The British designated this model as Mustang Mk.IA. The Mustang Mk IA was identical to the Mustang Mk I except that the wing-mounted machine guns were removed and replaced with four long-barrelled 20 mm (.79 in) Hispano Mk II cannon.
http://www.battle-fleet.com/pw/his/p51.htm