XB-38 – Don’t put all your Engines in One Basket

The XB-38 was born out of a necessity for diversification and redundancy in military aircraft production. In the early 1940s, the United States Army Air Forces (USAAF) recognised the strategic importance of the B-17 but also understood the risks associated with relying on a single-engine model, the Wright R-1820, used in the B-17.


Don’t put all your Engines in One Basket

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Lockheed, known for its engineering prowess, was tasked with creating a variant of the B-17 that would utilize a different power plant. This initiative aimed to ensure that, in the event of a shortage or sabotage of the Wright engines, the production and effectiveness of the US bomber fleet would not be significantly compromised.

Wow the XB-38 looks good!
Wow, this thing looks good!

Lockheed’s approach was innovative yet straightforward. They chose to replace the B-17’s original radial engines with the Allison V-1710 liquid-cooled V-12 engines, commonly used in fighter aircraft like the P-38 Lightning.

This choice was not just about diversifying the engine supply but also a gamble on performance enhancement. The Allison engines were more powerful and promised higher speeds, a crucial advantage in aerial warfare.

Wright R-1820 vs Allison V-1710


The comparison between the Wright R-1820 and the Allison V-1710 engines, both pivotal in the era of World War II aviation, involves contrasting their design philosophies, performance characteristics, and operational roles. These differences not only reflect the diverse engineering approaches of the time but also underscore how these engines shaped the aircraft they powered, notably the B-17 and its experimental variant, the XB-38.

The Wright R-1820 Cyclone was a radial engine, a design characterised by its cylinders arranged in a circle around the central crankcase.

The R-1820 powered a variety of early 30s aircraft.
The R-1820 was reliable and tough – perfect for a heavy bomber.

This air-cooled engine was robust, reliable, and relatively simple in terms of maintenance and repair, making it a preferred choice for many military aircraft, including the famed B-17 Flying Fortress. Its radial configuration offered a good power-to-weight ratio and was less prone to overheating, a critical advantage in the demanding conditions of wartime aviation.


In contrast, the Allison V-1710 was a liquid-cooled V-12 engine, reflecting a more complex and modern approach. The use of liquid cooling allowed for more efficient thermal management at higher power outputs, but it also introduced the need for radiators and additional systems, increasing complexity.

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This engine was more streamlined and aerodynamic compared to the bulky radial engines, making it a suitable choice for sleek fighter designs like the P-38 Lightning.

The Allison engine used to power several iconic fighters as well as the XB-38.
The Allison engine used to power several iconic fighters.

Performance-wise, the Allison V-1710 had a higher power output compared to the Wright R-1820. This translated into potentially higher speeds and improved performance at higher altitudes, a significant advantage in air-to-air combat. However, this increased power came with the cost of more delicate maintenance and a greater vulnerability to battle damage, especially due to the radiator system.

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The radial, while less powerful, offered a dependable and steady performance. Its air-cooled design was less susceptible to battle damage, as it didn’t rely on a radiator that could be punctured. The reliability and ruggedness of the R-1820 made it ideal for heavy, long-range bombers like the B-17, where durability and consistent performance over long missions were crucial.

Which was Better?

In their operational roles, these engines represented the differing needs of military aviation. The Wright R-1820’s reliability and ease of maintenance made it an excellent choice for the B-17, which required long-range capability and sustained performance under challenging conditions. The robust nature of the R-1820 ensured that the B-17 could endure substantial damage yet remain operational, a trait legendary among the bomber crews.

The B-17 was a tough bird and Allison engines would have compromised that.
The B-17 was a tough bird and Allison engines would have compromised that.

The Allison V-1710, with its higher performance capabilities, was more suited to fighter aircraft where speed, altitude performance, and manoeuvrability were paramount. However, when adapted to the XB-38, a variant of the B-17, the V-1710’s vulnerabilities became apparent. The engine’s increased maintenance needs and susceptibility to overheating and damage were less compatible with the demanding requirements of a heavy bomber.

From B-17 to XB-38

The conversion process from B-17 to XB-38 was far from simple. The Allison engines required significant modifications to the airframe. The sleeker, liquid-cooled engines altered the aircraft’s aerodynamics and demanded a complete redesign of the nacelles (engine housings).

Additionally, the cooling system for these engines was vastly different from the air-cooled radials. This necessitated the integration of radiators and ductwork, altering the bomber’s weight distribution and flight characteristics.

The engine cowling had to be significantly redesigned.
The engine cowling had to be significantly redesigned.

These changes, while theoretically beneficial, brought forth a series of challenges. The XB-38, although faster, experienced overheating problems, a common issue with liquid-cooled engines at the time. Furthermore, the alteration in weight and balance required pilots to adapt to different handling characteristics compared to the B-17.

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Fire Hazard

The XB-38 took its maiden flight in May 1943. Initial tests showed promise, with the aircraft achieving higher top speeds than the standard B-17. However, the joy was short-lived. During the ninth test flight, an engine caught fire, leading to an emergency landing.

The aircraft was destroyed, and the pilot narrowly escaped. This incident underscored the risks associated with the liquid-cooled engines, particularly in the demanding conditions of long-range bombing missions.

The End of the Line and Its Legacy

Following the loss of the prototype and considering the technical challenges, the USAAF decided not to proceed with the XB-38 program. The project was cancelled, and the focus shifted back to the proven B-17 design with its radial engines. The XB-38, therefore, remained a single-prototype experiment.

Initial test flights of the XB-38 were successful...
Initial test flights were successful…

Despite its brief existence, the XB-38 project contributed valuable lessons to aviation. It showcased the potential and limitations of engine interchangeability in existing airframes, a concept that would be revisited in later aircraft designs.

The XB-38 also highlighted the importance of reliability and ease of maintenance in military aircraft, factors that sometimes outweighed performance enhancements.

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The XB-38, though a mere blip in the vast timeline of aviation history, offers a fascinating glimpse into the challenges and innovations of wartime aircraft design. It reminds us that progress often comes through trial and error, and not all experiments lead to success.

However, each attempt, successful or not, paves the way for future advancements. The XB-38’s story is a tribute to the spirit of innovation and resilience in the face of adversity, qualities that continue to drive aviation forward.