The Douglas XTB2D Skypirate was a Second World War-era torpedo bomber designed by Douglas Aircraft Company for the U.S. Navy. It featured a powerful Pratt & Whitney R-4360 Wasp Major engine and could carry four torpedoes, an unprecedented payload for its time.
Only two were built, but they proved to possess excellent performance and capabilities. Sadly, in the flurry of new technologies in the post-war years, the XTB2D Skypirate has been long forgotten.
Contents
Background
Conceived during a time of intense innovation and rapid development, the Skypirate was the brainchild of the Douglas Aircraft Company, aimed at fulfilling a specific and demanding role within the United States Navy’s arsenal – the torpedo bomber.
The impetus for the Skypirate’s development came from the Navy’s 1942 request for an advanced torpedo bomber after the attack on Pearl Harbor in late 1941. The aircraft occupying that role at the time were the Douglas TBF Devastator, an early 1930s design, and the Grumman TBF Avenger, which first flew before the US had even entered WWII.
It was no easy task; this aircraft had to have unparalleled payload capacity and range. It was envisioned as the ultimate propeller-powered torpedo bomber, designed to carry a heavier payload than any of its contemporaries. With such a fast and long-range aircraft, the Navy could extend the range of a carrier task force so they could strike enemy ships long before they could be hit in return.
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It would also need to carry out conventional bombing missions if required.
Fortunately, US aircraft manufacturer Douglas had already been pondering a Devastator replacement for some time. In October 1943, the US Navy contracted Douglas to produce two prototypes for testing and evaluation.
However, the Navy wanted this aircraft to operate from its new Midway-class carriers. These carriers, larger and more capable than their predecessors, necessitated an aircraft that could leverage their expanded operational capacities, thus driving the need for a new, more powerful torpedo bomber.
Douglas’ proposal was named the TB2D Skypirate. Two prototypes were made and extensively tested, proving to have exceptional capacities for the role they were designed to fill.
Despite its promising design, the Skypirate’s development was marred by many challenges. Partway through the XTB2D’s development, the US Navy had essentially eliminated the Japanese carrier force threat, so there was a reduced need for such a capable torpedo bomber.
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Technological advancements and the eventual conclusion of the Second World War also contributed to shaping the fate of the Skypirate.
The XTB2D Skypirate
The Douglas XTB2D Skypirate was an ambitious aircraft design that sought to push the boundaries of what a torpedo bomber could achieve.
It had a rather conventional layout, with a stocky fuselage, a single engine at the nose, a cockpit, and a low-mono-wing, which had a distinct gull shape. Behind the cockpit was a gun turret. Another was situated in the belly of the aircraft, facing the rear. In total, it was operated by a crew of three.
As evident just by looking at it, the Skypirate had a very large airframe significantly larger than existing carrier-based bombers. This large size was necessary to accommodate its primary payload – up to four torpedoes – a powerful engine, and large quantities of fuel.
The capacity to carry multiple torpedoes was to allow the Skypirate to execute powerful, coordinated strikes against enemy naval targets. In fact, it could carry over four times the payload of a TBF Avenger.
Powering the Skypirate was the Pratt & Whitney R-4360 Wasp Major engine, one of the most advanced and powerful piston engines of its era. This 28-cylinder radial engine, often referred to as a “corncob” engine due to its unique four-row configuration, was capable of producing 3,000 horsepower.
The sheer power of the R-4360 enabled the Skypirate to achieve the necessary speed and range for long-distance missions, while also carrying heavy payloads. The engine’s robust performance was complemented by a supercharger, enhancing its capability at higher altitudes and ensuring that the Skypirate could operate effectively in various combat scenarios.
For defense, the aircraft was equipped with multiple machine guns strategically positioned to provide comprehensive coverage against enemy fighters. These included dorsal and ventral gun positions, as well as wing-mounted guns, which could be used for both offensive ground-attack missions and defensive air combat.
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Structurally, the Skypirate featured a reinforced tricycle undercarriage designed for the rigors of carrier operations. The landing gear was robust, with a wide stance to ensure stability during takeoffs and landings on the moving decks of aircraft carriers. While these features helped the aircraft, its size and weight posed challenges for carrier operations.
The Skypirate was designed to carry a variety of munitions beyond torpedoes, thanks to its four Mark 51 Mod 7 bomb racks. With these underwing racks, it could take conventional bombs, mines, depth charges, and more, making it adaptable for different types of missions.
As part of a US Navy requirement, the Skypirate was designed to be as easy to work on as possible. Equipment was strategically located to provide good access, and placed within the same areas so mechanics could remain near one section of the aircraft.
All in, the XTB2D Skypirate measured 46 ft (14 meters) in length and had a wingspan of 70 ft (21 meters). Its maximum take-off weight was over 17 tons! Despite its large size, its R-4360 engine allowed it to reach a top speed of 350 mph.
Big plane, big engine
The powerful Pratt & Whitney R-4360 Wasp Major engine was the heart of the XTB2D. This engine is the largest piston aircraft engine to enter production in the US. It was a critical component that enabled the Skypirate to meet its demanding performance requirements, providing the necessary power to carry heavy payloads over long distances that few other single engines could.
The R-4360 Wasp Major was a 28-cylinder radial engine arranged in four rows of seven cylinders, giving it a distinctive “corncob” appearance. This complex design allowed the engine to deliver an impressive 3,000 horsepower, making it one of the most powerful piston engines of its time.
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The engine’s displacement was 4,362 cubic inches (71.5 liters), which contributed to its ability to generate such high power output.
To put all this power to use, the XTB2D Skypirate was fitted with a contra-rotating propeller. This provided the blade area necessary to properly use the R-4360’s power while countering torque.
One of the key features of the R-4360 was its supercharger, which boosted the engine’s performance at higher altitudes. The supercharger compressed the incoming air, allowing the engine to produce more power by burning more fuel, even in the thin air of higher altitudes. This capability was particularly important for the Skypirate, as it needed to maintain high performance during long-range missions and while carrying heavy payloads.
However, the complex design of the R-4360 also meant that its reliability suffered. This was mainly related to overheating issues due to poor air circulation through the four rows of cylinders. As a result, it required meticulous maintenance and skilled technicians to keep it running smoothly.
The complexity of its four-row radial configuration was also challenging in terms of production, requiring a high level of expertise. This engine was truly pushing the limits of air-cooled piston technology and was the last hurrah before the introduction of the jet engine.
For the Skypirate, the R-4360 Wasp Major engine was instrumental in enabling the aircraft to achieve its ambitious design goals. The engine’s power allowed the Skypirate to carry four times the payload of the Avenger, making it a formidable weapon platform capable of delivering significant firepower in naval engagements.
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But in hindsight, the aircraft would have likely been better suited to a turboprop engine, which were in development at the same time as the Skypirate. Slightly later aircraft built to serve in similar roles were often equipped with turboprop engines.
Armament
The Douglas XTB2D Skypirate was designed with an impressive array of armaments, making it one of the most heavily armed torpedo bombers of its time. Its primary offensive capability revolved around its large bomb load that was a significant advancement over the one or two torpedoes typically carried by other torpedo bombers of the era.
This payload capacity allowed the Skypirate to deliver a concentrated and powerful strike against enemy naval targets, increasing the likelihood of crippling or sinking multiple ships in a single sortie. It also reduced the amount of aircraft needed per amount of ordnance released.
The radio operator/belly gunner served as the bombardier during bombing runs, using a Mark 15 Mod 5 bombsight. This was connected to a Sperry autopilot, which provided extremely smooth and delicate control over the aircraft during a run for the highest accuracy.
Defensive armament was another critical aspect of the Skypirate’s design. The aircraft was equipped with multiple machine guns strategically placed to provide defensive coverage. These included a dorsal (top) turret and a ventral (bottom) machine gun, which protected the aircraft from attacks coming from above and below. The belly position was located within a tunnel that protruded below the aircraft’s fuselage.
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Furthermore, the Skypirate had provisions for wing-mounted guns. These additional guns could serve dual purposes: they could be used for offensive ground-attack roles or for engaging enemy aircraft. This brought the aircraft up to a total of seven .50 caliber machine guns – not bad for a single-engine machine.
If needed, it could carry additional .50 caliber machine guns in two-gun pods mounted under the wings. These pods were able to charge the guns in the event of jams.
Purpose and Fate
The XTB2D carried out its first flight on March 13, 1945. The second and final version flew a few months later. Both aircraft were flown without any armament fitted, with the areas faired over.
Interestingly, neither Skypirate prototypes were built with the belly machine position or tunnel. This portion had always been designed to be detachable, but its complete omission on the prototypes has caused some to suspect that the aircraft’s role changed slightly during development.
It also alludes to the idea that the Skypirate could have had a “modular” aspect to its design, and the belly position could be swapped out with other mission-related equipment.
The Douglas XTB2D Skypirate was conceived to fulfill a vital role within the U.S. Navy. Its primary mission was to serve as a heavy torpedo bomber capable of delivering substantial payloads against enemy ships, a role that was becoming increasingly important in the context of naval warfare in the Pacific Theater.
It was designed to operate from the new Midway-class aircraft carriers, which were larger and more capable than their predecessors, and needed aircraft that could fully utilize their expanded operational capacities.
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The XTB2D’s role was envisioned to go beyond the capabilities of existing torpedo bombers like the TBF Avenger. While the Avenger had proven itself effective, the Navy recognized the need for an aircraft that could carry a heavier payload over longer distances.
Its ability to carry four torpedoes gave it much better striking power, allowing it to launch more coordinated and devastating attacks on enemy fleets. This increased payload capacity meant that the Skypirate could engage multiple targets in a single sortie, potentially altering the outcome of naval battles by crippling or sinking several enemy vessels.
In addition to its primary role as a torpedo bomber, the Skypirate was designed to be a versatile platform capable of performing a variety of secondary missions. Its wing racks could carry conventional bombs, depth charges, mines, machine guns, and incendiary bombs, making it adaptable for different combat scenarios.
This versatility would have allowed the Skypirate to undertake anti-submarine warfare, ground attack missions, and bombing runs, thereby extending its utility beyond torpedo attacks.
The development of the Skypirate also reflected a broader strategic shift in naval aviation. The introduction of the Midway-class carriers marked a move towards larger and more capable carrier strike groups.
However, the importance of the Skypirate diminished as the war progressed and new technologies and tactics emerged. The advent of more versatile and effective aircraft, and the destruction of Japan’s ability to mount significant aircraft carrier missions, meant there was no longer a need to push range out as far as possible.
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Additionally, the end of World War II brought about significant changes in military priorities and budgets. The cessation of hostilities led to a reduction in the demand for new aircraft, and many advanced projects were canceled or scaled back. The Skypirate, despite its promising design, fell victim to these post-war cutbacks. The program was canceled in 1946, and the Skypirate never progressed beyond the prototype stage.
