In early 1944, North American submitted a proposal to the Army Air Forces for an improved attack bomber, one which would provide the firepower of the B-25 strafers but with substantially improved performance. It was given the designation NA-98X by the company. Since it was not designed for any USAAF requirement, it never carried a USAAF designation. It was apparently intended as a lower-cost alternative to the heavily-armed Douglas A-26B Invader.
Power was to be provided by a pair of Pratt & Whitney R-2800 air-cooled radials, housed inside new low-drag cowlings and driving a pair of three-bladed propellers with large conical-shaped spinners. Armament improvements were to include a computing gunsight and a new North American-designed low-drag canopy for the top turret. A compensating sight was to be used in the tail turret, and illuminated reflector optical sights were to be used on the waist guns. The wing tips were square-cut rather than rounded, permitting the ailerons to be extended further outboard to provide better roll control.
The proposed aircraft was to be built in two different versions--a medium bomber version and a strafer version. As a strafer, a solid nose with eight 0.50-inch guns with 400 rpg was to be fitted. In addition, two 0.50-inch upper turret guns with 400 rpg, two 0.50-inch tail guns with 600 rpg, and two 0.50-inch waist guns with 200 rpg were to be carried. A second strafer option involved the addition of four guns mounted in blisters on the side of the fuselage, for a total of no less than 18 guns. As a medium bomber, the solid nose was replaced by a transparent nose containing a bombardier, two fixed nose guns with 300 rpg and one flexible nose gun with 300 rounds.
The 302nd B-25H (serial number 43-4406) was chosen as a testbed for the modifications. The new aircraft was powered by a pair of Pratt & Whitney R-2800-16 engines with Bendix Stromberg carburetors. Large conical propeller spinners were used, and high-speed inlet for the carburetors were added at the top of the engine cowlings. The wing tips were square cut. Except for the removal of the fuselage blister gun pack, the aircraft had the same armament fit (including the 75-mm cannon) as the B-25H.
The first flight of NA-98X took place on March 31, 1944, test pilot Joe Barton being at the controls. He reported better speed and acceleration, reduced vibration, and a higher roll rate. War emergency power could bring the aircraft to 10,000 in 4.9 minutes and in 5.3 minutes at military power. A maximum speed of 328 mph could be achieved at sea level with war emergency power.
It was recognized that the increased power of the R-2800 engine, acting in concert with the increased aileron area and reduced stick forces, might make it possible to operate the aircraft in performance regimes where excessive bending moments could be imposed on the wings, maybe even leading to a catastrophic failure and loss of the aircraft. Consequently, during flight testing, it was agreed that the maximum airspeed would be restricted to 340 mph and the normal acceleration would not exceed 2.67g.
On April 24, 1944, the NA-98X was taken up for a test flight by Maj. Perry Ritchie and Lt. Winton Wey. During a low speed pass over Mines Field, the aircraft disintegrated in mid-air and crashed, killing both pilots. An investigation showed that both outer wing panels had been ripped off the aircraft during the low-speed pass, the plane having been flown beyond its structural limitations by its crew. Following the crash, all further work on the NA-98X project was abandoned.