General Motors became a major player in the American manufacturing realm when World War 2 arrived for the United States. With its automotive plants adept at mass-production, it was only fitting that the US military look to such institutions for production of such war implements as bullets, guns, tanks and - of course - aircraft. General Motors made an easy conversion to production for the new orders (both foreign and domestic) and had already developed a hand in the aircraft industry with the ownership of the Allison engine company.
Allison was a key supplier of aircraft engines to the US Army and took to development of a new series product under the designation of V-3420, developed from the mating of two V-1710 series engines. In theory it was a vastly powerful engine but it remained wholly untested in the real world war time market. Nevertheless, a new product warranted equally new sales for the division and General Motors was not going to let its new engine development lay by the wayside - not with a world war going on.
By 1942, development of the war was not without its growing pains for the US military. Though it was already putting together a successful stead of legendary "pursuit" fighters in the North American P-51 Mustang, the Republic P-47 Thunderbolt and the Lockheed P-38 Lightning, these aircraft would generally not reach their true pinnacles for a few more years to come. As such, the need was apparent for better performing, high-flying and lethal interceptors to help stave off any aerial advantage that lay in the German Luftwaffe to this point. As such, a fighter needed to be developed that could provide a lethal punch against both fighter or bomber and could respond to incoming threats through excellent climbing abilities and top speed.
General Motors saw this as the perfect opportunity to showcase the new Allison powerplant and moved to convert their Fisher Body Plant in Cleveland, Ohio for production of not only the Allison engine but also of a new interceptor to fill the US Army need. Accordingly, they penciled out a design specifically fitting their powerplant and submitted it for review. Amazingly, the US Army took notice and accepted the General Motors idea - moreso in terms that the new fighter could be rapidly produced while fulfilling the performance requests as needed by the Army. The contract called for two complete prototypes and the designation of XP-75 was assigned.
To keep design and development of the new system short, the idea presented was to build an aircraft out of proven portions of existing airframes with the entire concoction fitted around the Allison V-3420. Outer wing surfaces were pulled from a P-51 Mustang while the wings themselves were originally slated to resemble the inverted "gull wing" implements as found on the Vought F4U Corsair fighter. The inverted gull wings were dropped from the design and, instead, the wings of a Curtiss P-40 Warhawk was used in its place though the undercarriage of the F4U was still selected for use. The empennage was to be made up from the system of a Douglas Dauntless dive bomber.
The final result became one of the most unique - if not dysfunctional-looking - aircraft designs to emerge from World War 2. The fuselage became a long slender affair with the cockpit situated quite well-ahead of the low-mounted monoplane wings. The nose protruded out some and was capped with a contra-rotating set of three-bladed propellers (giving the appearance of six blades in the nose). The design called for the Allison powerplant to be fitted at the midway point of the fuselage with a drive shaft running under the cockpit floor and attached to each propeller system. As a liquid-cooled engine, the V-3420 had no need for air cooling scoops and could be fitted as such. The radiator was fed via two scoops mounted under the rearward portion of the fuselage. The empennage maintained curved surface features and was topped with a smallish sort of vertical tail fin and accompanying horizontal planes. The undercarriage consisted of two main landing gears - one mounted under each wing - and a diminutive tail wheel. The canopy was heavily framed at first and offered up adequate views all-around the airframe. In whole, the XP-75 was constructed over in all-metal stressed skin.
As with most other "X" plane design attempts emerging in World War 2, armament played an increasingly vital role in terms of lethality. American warplanes tended to favor the proven and high rate-of-fire inherent in their Browning air-cooled .50 caliber machine guns, almost always mounted in pairs in the wings or upper forward fuselage. The other world powers of the time preferred to work around the heavier hitting power of their 20mm, 30mm and 37mm cannons instead, albeit with slower rates of fire and complimented by machine guns. As such, the Fisher XP-76 would sport 6 x .50 caliber machine guns in her wings (the wings belonging to the P-40 Warhawk) and an additional 4 x .50 caliber machine guns in her upper forward fuselage. This ten-machine gun armament would have made a single burst from the XP-75 something to behold, however, fate would see to it that none of the proceeding XP-75 (or production P-75s) would have their armament installed.
As production geared up for the aircraft, General Motors found themselves caught up in the patriotic fervor gripping American companies. In response, the designation name of "Eagle" was now being applied to the XP-75. The XP-75 was completed and went airborne on November 17, 1943.
First results of the XP-75 in the air were far from what the US Army, or General Motors - or Allison for that matter - envisioned. The system proved highly underpowered and sluggish at the controls. Overall performance was nothing close to what was advertised and all looked to the Allison powerplant as the culprit. The second completed prototype joined the first in the air shortly thereafter with similar results from her Allison engine.
As wartime development goes, the US Army now changed their stance on the product at hand. Whereas the original requirement of 1942 stated a need for a new pursuit fighter fitting the interceptor mold, advances in the air and ground war led to a new need for a long-range "penetration" fighter in 1943 instead. Aircraft would now need the long range capabilities to escort bombers deep within enemy-held territories in their defense and leave the formations to tangle with equally adept enemy fighters. The Army, nevertheless, saw some hope in the XP-75 and put in an order for 2,500 P-75 Eagle production examples.
Knowing the specification had changed their plans altogether, Fisher put forth an new contract for six additional prototypes to fulfill the changed Army requirement. The contract was granted and work immediately began on evolving the base XP-75 design. A revised Allison powerplant emerged under the designation of V-3420-23 and promised improved performance and output. The framed canopy of old was also revised into a more modern "bubble" type assembly, offering vastly increased viewing angles from the pilot's seat. To fulfill the long-range qualities of a penetration fighter, additional fuel tanks were added. A new empennage was installed with a larger vertical tail fin and associated rudder plane.
The six long-range XP-75 Eagles were ready for further testing and review. These systems fared just as well as their preceding forms and fell well below expectations. As a fighter, the XP-75 was simply too big an aircraft with a low-powered engine to be of much use. Her maneuverability and handling suffered just as much to make the XP-75 virtually useless in any field of combat.
Production on other proven and more adept systems was well underway by 1944 and the war, for the most part, had begun to turn in the favor of the Allies on many fronts. As such, the XP-75 program was cancelled in full by the US Army on October 6th, 1944, putting an end to the failed Eagle program once and for all. Despite its cancellation, the Army allowed construction of six production P-75's (P-75A-1-GC) to continue for the sole purpose of trialing the Allison powerplant for possible future use.
Testing of the engine proved a limited endeavor resulting in the loss of the first prototype to a crash. the rest were left to their own fate and only one example survived the scrap heap to become a museum display piece (fully restored) at the Museum of the USAF in Dayton, Ohio.
Fisher assigned the following performance specifications to their aircraft. Many of these numbers were estimates and never achieved with the troublesome Allison V-3420-23 (rated at 2,600 horsepower): Maximum speed of 404 miles per hour with a cruising speed of 250 miles per hour. A service ceiling of 40,000 feet with a rate-of-climb equaling 4,200 feet-per-minute. Range was anticipated at 1,150 miles. An empty weight of 11,255lbs was listed as was a loaded weight of 19,420lbs.
The initial two prototypes were given serial numbers 43-46950 and 43-46951. The long-range XP-75s were given s/n 44-32161, 44-32162, 44-32163, 44-32164, 44-32165 and 44-32166. The six production Eagles became s/n 44-44549, 44-44550, 44-44551, 44-44552, 44-44553 and 44-44554.