P-75 Eagle – The Unstable Interceptor

The Fisher P-75 Eagle was developed as an interceptor aircraft in response to a specification by the United States Army Air Force (USAAF) for a plane with a rapid rate of climb that also had a high performance and maneuverability. Rather than modify an existing fighter, the USAAF decided to take a chance with a new concept design.

The task was given to Fisher Body, a subsidiary of the car manufacturing giant General Motors and a major manufacturer of Sherman tanks. Following the entrance of the United States into the war, many industrial corporations which had catered to the civilian market were given new instructions to focus on building aircraft for the American military.

Fisher Body had already established milestones within the automotive designing sector and sought to apply this experience to a new interceptor aircraft.

Their P-75 Eagle was produced to intense media interest and high expectations. However, both Fisher Body and the USAAF’s gamble on a new concept did not pay off, as the P-75 was plagued with various technical and design flaws. Despite modifications, the USAAF chose to abandon the idea of finding a new concept and focus on existing fighters.

Background

In September 1942, the United States Army Air Force wanted to boost its fighter and interceptor capabilities as the Second World War raged in Europe and the Pacific. The commercial industrial manufacturing sector was instructed by the government to shift their priorities to help produce military equipment for the American war effort.

One such example was Fisher Body Division which was a subsidiary of the car manufacturer General Motors. Fisher Body had helped to develop certain pioneering ideas within the car industry, including slanted windscreens and duel windscreen wipers in the 1930s.

Fisher Body Plant.
Completed horizontal stabilisers for B-29s at Fisher’s plant in Cleveland, Ohio.

After America entered the war, the company helped to build airframes for the B-25 Mitchell and assisted with the production and assembly of 200 units of the Boeing B-29 Superfortress bomber. Fisher Body began to research and produce their own aircraft designs during the war, hoping to build on their experience with experimental ideas and combining it with making use of existing aircraft designs.

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The company responded to the USAAF’s specification, which called for the development of a new fighter that had a higher rate of climb than any in production at the time that could act as an interception plane to shoot down enemy bombers. The new plane would also be built around the liquid cooled Allison V-3420 which was the most powerful aircraft engine in existence at the time.

Allison V-3420 engine at the National Air and Space Museum.
The Allison V-3420 was a 24 cylinder, 56 litre (3,420 cu in) double V engine, essentially made from two V12 connected together. It produced over 2,500 hp. Image courtesy of the Smithsonian National Air and Space Museum.

Development

In October 1942, the USAAF awarded Fisher Body a contract to produce two new prototypes of a new aircraft design.

Fisher Body sought to use their expertise in designing and manufacturing experimental car ideas to build and deliver the new design within a short timeframe. To accelerate the production process further, parts from existing aircraft were also sourced: the outer wing panels from the P-40, the tail assembly from the Douglas A-24 and the undercarriage from the Vought F4U Corsair were all taken and applied to the new plane.

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As the Fisher Body design team set to work, some of the parts were found to be less suitable than others and the decision was made to swap the P-40 wing panels to ones from the P-51 Mustang.

P-75 prototypes under construction by Fisher.
P-75 prototypes under construction by Fisher.

As per the specification, the new plane was built around the Allison engine unit which would drive contra (opposite) rotating propellers set one behind the other and with a total of six blades. The engine was placed in the middle of the fuselage rather than in the nose, and drove two shafts that were linked to the propeller.

During the prototype’s development, the USAAF updated its requirements in 1943 in response to an urgent need for escorting and fighter aircraft. The USAAF were now conducting bombing raids over Europe and in the pacific theatre with large bomber aircraft that required fighter escorts after USAAF crews noted that unprotected bomber formations were highly vulnerable.

The threat of Luftwaffe aircraft bombing campaigns against Britain were steadily becoming diminished too, thus reducing the need for interception aircraft for either the British export market or for the US military.

The last XP-75 to be built.
The last P-75 to be built.

American military strategists decided that escort fighters which could accompany bombers on their sorties were now of a higher priority than interceptors. Fisher Body responded by modifying the existing P-75 prototype designs to make them suited for long range fighting and mission escort roles. The USAAF responding by changing their order for the Fisher Body design and expanding it to 2,500 units with the understanding the project could be cancelled if the post-modification plane was found to be unsatisfactory in tests.

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Although General Motors were busy focusing on producing aircraft on the behalf of other manufacturers, including the Grumman TBF Avenger by this stage, the P-75 was given higher priority by the USAAF to alleviate the company from having to produce vast quantities of the B-29 Superfortress and to encourage design competition.

At the end of the design process, the P-75 was given the name “Eagle” and was promoted by the press as a new super plane.

P-75 nose.
A good view of the P-75’s contra-rotating propellers. They were driven by two shafts from the centrally mounted engine. This arrangement would crop up again in turboprop aircraft like the XF-84H Thunderscreech.

The P-75’s proposed armaments consisted of six 12 mm machine guns mounted in the wings and another four mounted at the front of the fuselage. The projected maximum speed was 433 miles per hour with a service ceiling of around 37,000 feet. The range was expected to be 2,000 miles.

The first design was also fitted with a traditional “greenhouse” cockpit canopy.

With the media fanfare and high expectations, Fisher Body submitted the P-75 Eagle for testing and proving trials in 1943.

Testing

The P-75 was sent for its maiden flight on the 17th of November, 1943, roughly a year after the first prototypes were built.

Despite the initial enthusiastic promotion of the aircraft, the prototype was found to have a number of serious issues during its first and subsequent test flights.

USAAF test pilots noted the stability of the plane was critically not up to standard, with the weight distribution and centre of mass off balance and hampering the performance. Poor spin characteristics were found as a result of this and the plane’s aerodynamics were deemed to be concerningly imbalanced.

Although the Allison engine was marketed as advanced, it was underpowered when fitted to the P-75, falling below its projected speed and horsepower range. It also had severe problems with overheating and its coolant system failing during flight.

The XP-75's side.
The P-75 was a bizarre looking aircraft, mostly in part due to its centrally-mounted engine.

Test pilots also concluded the ailerons would underperform and display issues when the P-75 was flown at high speeds. The general consensus after the initial proving runs was that the aircraft was in need of a substantial redesign in order to be safe to fly and meet USAAF standards.

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The design team at Fisher Body took the Eagle back to the drawing board and worked hard on redesigning the plane. The Eagle’s tail shape and size were modified and work was also done to address the engine’s habit of overheating. A new bubble glass canopy was added to the cockpit to replace the older greenhouse style canopy for improved streamlining and visibility for the pilot.

The improved version of the Eagle was resubmitted for USAAF tests in September 1944.

P-75 with early canopy.
Initially the P-75 was fitted with a heavily framed canopy. This would eventually be replaced by a bubble canopy.

However, by this stage the USAAF had started to lose interest in the project due to its delays and another change in priorities from the US military high command. A number of American military chiefs had decided by 1944 that it would be more cost effective and helpful for the constant demands of the US war effort to continue modifying existing fighter aircraft rather than produce an entirely new and potentially expensive plane from scratch.

By this stage, the twin engine Lockheed P-38 Lightning and the North American P-51 Mustang had already demonstrated themselves to have strong capabilities and active service showed the Mustang was an effective long range aircraft that met the USAAF’s escort requirements. The P-51 played a crucial role in helping the Allies to achieve air superiority in Europe and the tactical need to replace or supplement it with another fighter was considered redundant.

The USAAF made the decision to limit new aircraft production just as the newer version of the P-75 was ready, and on the 6th of October 1944, USAAF chiefs terminated all further production of the Eagle and keep it as an experimental aircraft only.

P-75 top view.
This angle clearly shows the P-51-type wings used on the P-75. Also, note the exhaust ports located near the centre of the fuselage.

As six units of the P-75 had already been produced by Fisher Body and two more were due to roll off the production line, the USAAF opted to retain them as test and research aircraft for the Allison V-3420 engine.

Fate

The P-75 briefly remained in experimental service as a test bed for the Allison unit, but no further models were produced and it was never used in an active combat situation after the USAAF pulled the plug on mass production.

Fourteen different prototype models of the P-75 were eventually produced in total, but out of all the airframes built by Fisher, three were written off in flying accidents during test runs while most of the other units were dismantled for spares and scrap.

One surviving P-57 Eagle has been preserved for public display at the National Museum of the United States Air Force in Dayton, Ohio. It was initially displayed in the experimental section of the museum before being removed for restoration in 1999 when rust was detected in the fuselage.

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At the time of writing, the restoration has been complete and the Eagle is back on display.

Guy Davey

I have a significant interest in aviation and military history, having graduated with a Master's degree in modern history at Oxford Brookes University in the United Kingdom. I have worked variously in the education and private business sectors. Outside of work, I enjoy both factual and fiction writing as a hobby. I currently live on the west coast of America and have been working towards obtaining my private pilot's licence.