April 1942 marked the first order of any P-51 variant by the US Army. 500 NA-97s were ordered. First named "Apache" by the Army, it was also called "Invader" and later mostly just "Mustang". The U.S. serials were 42-83663 to 42-84162.

The design was very similar to the Mustang I and P-51A. The powerplant was the Allison V-1710-87. The A-36 was a dive-bomber so the addition of hard points for two 500 lb bombs and dive brakes to slow the fast acceleration of the P-51 in a dive were added. The belly scoop was now fixed at the front. Armament was 2 guns in the nose and 4 in the wings, all .50 calibur.

The dive brakes were operated hydraulically and located on the top and bottom of each wing outboard of the guns. Plan was that they limit the dive speed to 250 mph but in practice the angle of dive was reduced to 70 degrees because the high stress of pull-out from a higher angle.

The hard points for the bombs were also capable of holding 75 gallon drop tanks for extended range. The top speed of the A-36 was down from added weight to about 358 mph at 5,000 feet (without external stores).

Only one A-36 was sent to the RAF for evaluation as A-36 EW998 in March 1943. The US used the A-36 in the Mediterranean theatre with first deliveries beginning in early 1943. When not in the ground attack role, the A-36 was essentially a low-altitude P-51A and was used as a fighter. The A-36 scored 101 air-air victories during WWII.

The A-36 proved to be a very stable platform for accurate weapons delivery. 177 were lost in action mostly due to the dangerous mission of low-level operations.