North American Aviation NA-98X Super Strafer

By William Pearce

In 1943, North American Aviation (NAA) created an internal design for an improved attack bomber that would provide the firepower of the B-25H but with substantially improved performance. This evolution of the B-25 line was intended as an alternative to the heavily-armed and delayed Douglas A-26B Invader. Power was to be provided by a pair of Pratt & Whitney R-2800 air-cooled radial engines housed inside low-drag cowlings and driving a pair of cuffed, four-blade propellers with spinners. The empennage was changed to a conventional single-tail, altering one of the B-25’s most notable characteristics. The wing tips were square-cut like a P-51’s, rather than rounded, permitting the ailerons to be extended farther outboard to provide better roll control. Armament improvements were to include a computing gun sight and a new low-drag canopy designed by North American for the top turret. A compensating sight was to be used in the tail turret and illuminated reflector optical sights for the waist guns. Otherwise, the aircraft had the same armament as the B-25H, including the 75mm cannon.

NAA B-25 based high performance attack bomber drawn by Eugene Clay.

In early 1944, a low-cost and less ambitious alternative was submitted to the Army Air Forces. This proposal was to take the existing B-25 airframe and apply many of the enhancements from the NAA internal design. A B-25H-5 (serial number 43-4406) was chosen as a testbed for the modifications, which no longer included the single tail and four-blade propellers. It was given the designation NA-98X by NAA and nicknamed Super Strafer. Since it was not designed for any USAAF requirement, it never carried an official USAAF designation.

The new aircraft was powered by a pair of 2,000 hp (1,491 kW) Pratt & Whitney R-2800-51 engines with 15 minutes of water injection, all housed in A-26 cowlings. Large conical spinners were used on the three-blade propellers. The squared wing tips allowed the ailerons to be extended by one foot, and the control system was changed to lighten the stick forces. Except for the removal of the fuselage blister gun packs, the aircraft had the same armament as the B-25H.

The NA-98X: a B-25H, serial number 43-4406, modified with R-2800 engines, spinners, and squared wings.

The first flight of the NA-98X took place at Mines Field in Los Angeles on 31 March 1944. NAA test pilot Joe Barton was at the controls with Jim Talman as the NAA flight engineer. The flight lasted an hour, and Barton reported better speed and acceleration, reduced vibration, and a higher roll rate compared to a standard B-25. The Super Strafer performed well. The aircraft reached 10,000 feet (3,048 m) in 4.9 minutes at war emergency power and in 5.3 minutes at military power. A maximum speed of 328 mph (528 km/h) was achieved at sea level with war emergency power, and Barton eventually achieved 350 mph (563 km/h) at a higher altitude. The performance numbers were a drastic improvement over the standard B-25H’s top speed of 273 mph (439 km/h) and 790 fpm (4.0 m/s) climb rate. There were 16 more NAA test flights before the aircraft was turned over to the USAAF for evaluation.

The increased power of the R-2800 engines, along with the increased aileron area and reduced stick forces, created the possibility to operate the aircraft outside the structural limits of the wings. This could lead to a catastrophic failure, loss of the aircraft, and possible loss of life. Without strengthening the wings for intended dives of 400 mph (644 km/h) and high-g pullouts, the maximum airspeed was restricted to 340 mph (547 km/h), and a g-limit of 2.67g was imposed during flight tests.

Front view of the NA-98X Super Strafer with the 75mm cannon and squared wingtips clearly visible.

Major Perry Ritchie was assigned as the evaluator for the USAAF and had made 13 flights in the aircraft before disaster struck on 24 April 1944. Returning from the aircraft’s 29th test flight, Maj. Ritchie and Lt. Winton Wey approached Mines Field at a very high speed, estimated to be above the 340 mph (547 km/h) red line. As he made a steep pull-up, in excess of the 2.67g restriction, both wings separated outboard the engine nacelles and struck the horizontal stabilizers, causing the tail to break free from the aircraft. The NA-98X crashed, killing both Maj. Ritchie and Lt. Wey. Maj. Ritchie had made this maneuver before and was warned not to do it again. Simply put, the aircraft was repeatedly flown beyond its known structural limitations by Maj. Ritchie. Following the crash, all further work on the NA-98X project was abandoned even though the RAF had shown interest after a test flight. Admittedly, the wings would have required a redesign to cope with the power from the R-2800s, negating any cost savings for performance on par with the Douglas A-26.

In its short 25-day life, the NA-98X Super Strafer totaled 40:15 hours of flight tests. NAA time totaled 21:25 hours over a period of 22 days, including eight days of down time. Total time for Maj. Ritchie was 18:50 hours over a period of three days.

Sources:
North American B-25 Variant BriefingWings of Fame Volume 3  (1996)
American Bomber Aircraft Development in World War 2 by Bill Norton (2012)
North American Aircraft 1934-1998 Volume 1 by Tom Avery (1998)
http://www.joebaugher.com/usaf_bombers/b25_16.html
http://www.warbirdinformationexchange.org/phpBB3/viewtopic.php?t=31406&view=previous

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