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Republic XP-72 Ultrabolt

Prototype High-Altitude Fighter / Interceptor

United States | 1944

"The Republic XP-72 Ultrabolt would have been an exceptional fighter but the Allied advance in both Europe and the Pacific derailed its promising development."

Authored By: Staff Writer | Last Edited: 03/31/2019 | Content ©www.MilitaryFactory.com | The following text is exclusive to this site; No A.I. was used in the generation of this content.
The Republic XP-72 Wasp Major-powered "Super Thunderbolt" (or "Ultrabolt") was being developed in the latter part of World War 2 (1939-1945) as the ultimate incarnation of the war-winning P-47 Thunderbolt fighter/fighter-bomber (also produced by Republic). The XP-72 incorporated largely the same airframe with the massive, experimental Pratt & Whitney R-4360 "Wasp Major" series engine rated at 3,450 horsepower. As promising as the high-altitude fighter was becoming, the changing nature of the war ultimately found little need for such an aircraft. No more than two prototypes were produced.

As an expediency, the XP-72 made use of both the proven fuselage and wings of the P-47 before it. Some alterations to accommodate the large Pratt & Whitney were made to the airframe and this included a revised lower cowling to allow for proper aspiration of the engine. A large-diameter, four-bladed propeller unit was affixed to the powerplant. The aircraft was indeed considered the "Super Thunderbolt" when compared to the original P-47 offering.

Developed as soon as the P-47 was entering service, the Republic team focused on two preliminary designs that were to take the beneficial design qualities of the existing P-47 and improve upon it wherever possible. The United States Army Air Force (USAAF) accepted both initial designs, one being an XP-69 mated with a Wright R-2160 radial engine and the other the XP-72 with its Pratt & Whitney. As the designs progressed, the XP-69 design - complete with high-altitude features including a pressurized cockpit and armament of 2 x 37mm cannons and 4 x 12.7mm machine guns - was dropped on May 11th, 1943, in favor of the more promising XP-72. Two XP-72 prototypes were then ordered on June 18th, 1943 with the air war in full swing over both the Pacific and Europe.

The first XP-72 prototype flew on February 2nd, 1944 (with a basic four-blade propeller) and development was very much unlike other aircraft of the period in that the program actually exhibited few delays as it progressed. The first prototype flew through the air with ease - even with the turbocharger off - at a very exceptional speed of 480 miles per hour (though neither XP-72 prototype would ever exceeded 500 miles per hour in flight testing). The aircraft proved quite responsive at the controls and pilots commented on it as being "a joy to fly". With the XP-72 achieving success in its early development, the aircraft was envisioned to rival the high-speed, high-altitude performance found in the latter series of British Supermarine "Spitfire" fighters while at the same time offering the operational range that the Spitfire lacked. Indeed, the XP-72 might have been a good candidate for V-1 rocket interception duty over the South of England than the Spitfires already charged with the task.

The second prototype was given an Aero Products contra-rotating propeller arrangement (a pairing of three-bladed units to a single engine, equaling six total blades) and an estimated maximum speed of 550 miles per hour. Primary armament was to become an array of 6 x 12.7mm (.50 caliber) machine guns mounted in the wings (a reduction from the 8 x 12.7mm machine guns in the original P-47 Thunderbol). An alternative weapons loadout built into the contract was to allowed for 4 x 37mm autocannons in place of all of the machine guns. As secondary armament, the aircraft was to carry high-explosive rockets, 2 x 1,000lbs bombs, or 2 x fuel droptanks as needed at hardpoints under each wing.

Despite the performance gains and relatively easy development process, the conditions in the latter years of the war were such that all-new high-altitude fighters / interceptors were in little demand as existing types were more than enough to fulfill the role. The simple truth became that the XP-72 was no longer a needed commodity in the grand scope of the war - particularly with prop-powered aircraft reaching their technological apexes and turbojet technology beginning to take hold. As such, the two XP-72 prototypes were dropped from further development and scrapped by war's end along with cancellation of a production contract for 100 of the type. The XP-72 "Super Thunderbolt" would go down as just another of World War 2's "what-if" programs that failed to materialize any tangible fruits by war's end - leaving to the imagination what impact, if any, such powerful aircraft would have had if promptly adopted into operational service.

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Power & Performance
Those special qualities that separate one aircraft design from another. Performance specifications presented assume optimal operating conditions for the Republic XP-72 Ultrabolt Prototype High-Altitude Fighter / Interceptor.
1 x Pratt & Whitney R-4360-13 "Wasp Major" 28-cylinder, 4-row, air-cooled radial piston engine developing 3,450 horsepower and driving a four-bladed propeller unit at the nose.
490 mph
788 kph | 425 kts
Max Speed
41,998 ft
12,801 m | 8 miles
Service Ceiling
1,200 miles
1,931 km | 1,043 nm
Operational Range
5,000 ft/min
1,524 m/min
City-to-City Ranges
Operational range when compared to distances between major cities (in KM).
The nose-to-tail, wingtip-to-wingtip physical qualities of the Republic XP-72 Ultrabolt Prototype High-Altitude Fighter / Interceptor.
36.6 ft
11.15 m
O/A Length
40.9 ft
(12.47 m)
O/A Width
16.0 ft
(4.88 m)
O/A Height
11,475 lb
(5,205 kg)
Empty Weight
17,494 lb
(7,935 kg)
Design Balance
The three qualities reflected below are altitude, speed, and range. The more full the box, the more balanced the design.
Available supported armament and special-mission equipment featured in the design of the Republic XP-72 Ultrabolt Prototype High-Altitude Fighter / Interceptor .
6 x 12.7mm Browning M2 Heavy Machine Guns (HMGs) in wings.

4 x 37mm Automatic cannons in wings.

2 x 1,000lb bombs, Rockets, OR Fuel Drop tanks under the wings.
Notable series variants as part of the Republic XP-72 Ultrabolt family line.
P-47 "Thunderbolt" - Production Series Designation of base P-47 design on which the XP-72 was built upon; 8 x 12.7mm machine guns.
XP-72 "Ultrabolt" - Prototype Series Designation; two examples constructed; 6 x 12.7mm machine guns; 1 x Pratt & Whitney R-4360-13 Wasp Major engine of 3,450 horsepower.
Global customers who have evaluated and/or operated the Republic XP-72 Ultrabolt. Nations are displayed by flag, each linked to their respective national aircraft listing.

Total Production: 2 Units

Contractor(s): Republic Aviation - USA
National flag of the United States

[ United States (cancelled) ]
Relative Max Speed
Hi: 500mph
Lo: 250mph
Aircraft Max Listed Speed (490mph).

Graph Average of 375 MPH.
Era Crossover
Pie graph section
Showcasing Aircraft Era Crossover (if any)
Max Alt Visualization
Small airplane graphic
Production Comparison
Entry compared against Ilyushin IL-2 (military) and Cessna 172 (civilian) total production.
MACH Regime (Sonic)
RANGES (MPH) Subsonic: <614mph | Transonic: 614-921 | Supersonic: 921-3836 | Hypersonic: 3836-7673 | Hi-Hypersonic: 7673-19180 | Reentry: >19030
Aviation Timeline
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Image of the Republic XP-72 Ultrabolt

Mission Roles
Some designs are single-minded in their approach while others offer a more versatile solution to airborne requirements.
Some designs stand the test of time while others are doomed to never advance beyond the drawing board; let history be their judge.
Going Further...
The Republic XP-72 Ultrabolt Prototype High-Altitude Fighter / Interceptor appears in the following collections:
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