The Sikorsky XBLR-3 was one of the most ambitious American bomber projects of the 1930s. Although it never advanced beyond the design and mock-up stage, the aircraft remains a fascinating chapter in military aviation history. Developed by Sikorsky Aircraft as part of the United States Army Air Corps' secret long-range bomber program, the XBLR-3 represented an early attempt to create an intercontinental heavy bomber capable of unprecedented range and endurance.
Today, the Sikorsky XBLR-3 is remembered as a visionary aircraft concept that showcased advanced engineering ideas long before they became standard in strategic bomber design.
Sikorsky XBLR-3
What Was the Sikorsky XBLR-3?
The Sikorsky XBLR-3 was an experimental long-range heavy bomber proposed in 1935 for the United States Army Air Corps' classified “Project D” competition. The program sought to explore the future of strategic bombing by evaluating aircraft capable of carrying heavy payloads over extremely long distances.
Sikorsky entered the competition alongside two other major American aircraft manufacturers:
Boeing XBLR-1 (later developed into the XB-15)
Douglas XBLR-2 (which evolved into the XB-19)
Sikorsky XBLR-3
The Army Air Corps ultimately selected Boeing and Douglas designs for further development, while the Sikorsky proposal was canceled in March 1936 before a prototype could be built.
Origins of Project D
In February 1935, the U.S. Army Air Corps launched a secret study aimed at creating a new category of aircraft known as BLR (Bomber, Long Range). The objective was not immediate mass production but rather an exploration of future military aviation capabilities.
At the time, military planners were increasingly interested in aircraft that could strike targets at unprecedented distances without relying on overseas bases. The XBLR-3 was Sikorsky’s response to this challenge.
Design and Development
Although Sikorsky was primarily known for its successful flying boats such as the S-40 and S-42, the company decided to apply its expertise in large aircraft design to the XBLR-3 project.
Early Concept
The original XBLR-3 concept was highly unconventional. Early design studies featured:
Twin-boom construction
Mixed tractor and pusher propeller arrangements
Multiple defensive gun positions
Extremely long-range capability
The concept resembled the futuristic military aircraft often depicted in aviation magazines and pulp fiction illustrations of the era. As development progressed, the design evolved into a more conventional four-engine heavy bomber.
Advanced Engineering Features
The final proposal incorporated several advanced features for its time:
All-metal construction
Low-wing monoplane configuration
Tricycle landing gear with nose wheel
Retractable undercarriage
Large internal fuel capacity
Extensive wing flaps for improved lift
Many of these features would later become standard in large military aircraft.
Engine Configuration
One of the most impressive aspects of the XBLR-3 was its planned powerplant arrangement.
The aircraft was designed to use four Allison XV-3420 liquid-cooled engines. Each engine produced approximately 1,600 horsepower and drove a large metal adjustable-pitch propeller.
The Allison XV-3420 itself was a remarkable engine, essentially combining two Allison V-1710 engines into a single 24-cylinder power unit. The same engine family was proposed for other Project D bomber competitors as well.
Sikorsky XBLR-3 Specifications
Dimensions
Specification
Measurement
Wingspan
204.9 ft (62.45 m)
Length
120 ft (36.58 m)
Height
35 ft (10.67 m)
Performance Estimates
Specification
Measurement
Maximum Speed
221 mph (355 km/h)
Cruising Speed
127 mph (205 km/h)
Maximum Range
7,652 miles (12,312 km)
Flight Endurance
62 hours
Maximum Takeoff Weight
120,000 lb (54,422 kg)
These figures were extraordinary for the mid-1930s and reflected the strategic ambitions behind Project D.
Why the XBLR-3 Was Canceled
Despite its innovative design, the XBLR-3 faced several challenges.
Sikorsky lacked extensive experience building very large land-based bombers. While the company had achieved considerable success with flying boats, strategic bombers required different structural and operational considerations.
In March 1936, Army Air Corps evaluators reviewed wooden mock-ups from competing manufacturers. The Douglas XBLR-2 received higher evaluations, and budget constraints prevented simultaneous development of multiple giant bomber projects. As a result, the Sikorsky XBLR-3 program was terminated.
Wind Tunnel Testing and Research
Although the bomber never reached prototype construction, Sikorsky produced a detailed 1/25-scale wooden model for aerodynamic evaluation.
The model underwent testing at the NACA Langley Research Center, one of America's leading aeronautical research facilities. These studies helped validate portions of the aircraft's aerodynamic design and contributed to Sikorsky's broader engineering knowledge.
Influence on Later Aircraft
The XBLR-3 project was not entirely wasted. Several design concepts developed during the bomber program were later incorporated into other Sikorsky aircraft.
Engineers applied lessons from the project to the development of:
Sikorsky XPBS-1 patrol bomber flying boat
Sikorsky VS-44 commercial flying boat
Later large-aircraft structural designs
Some historians consider the XBLR-3 an important stepping stone between Sikorsky's famous flying boats and the company's eventual transition into helicopter development.
The Last Fixed-Wing Bomber Vision from Sikorsky
The XBLR-3 occupies a unique place in aviation history because it was among the final fixed-wing military aircraft projects pursued by Sikorsky.
In the years that followed, the company increasingly focused on rotorcraft technology. This strategic shift ultimately led to Sikorsky becoming one of the world's most influential helicopter manufacturers and the creator of many groundbreaking helicopter designs.
Legacy of the Sikorsky XBLR-3
Although it never flew, the Sikorsky XBLR-3 remains an intriguing example of pre-World War II aviation ambition. The aircraft represented a bold attempt to push the boundaries of range, endurance, and strategic capability at a time when long-distance bombing was still largely theoretical.
Its cancellation illustrates the intense competition among American aircraft manufacturers during the 1930s and highlights how many innovative concepts never advanced beyond the drawing board.
Today, the XBLR-3 survives only through photographs, engineering records, and scale models, yet it continues to fascinate military aviation historians and aircraft enthusiasts.
Conclusion
The Sikorsky XBLR-3 was a visionary heavy bomber project that embodied the technological optimism of the 1930s. Designed for the secretive Project D competition, it featured advanced engineering, remarkable projected performance, and innovative ideas that were ahead of their time. While the aircraft never entered production, its influence can still be traced through later Sikorsky designs and the broader evolution of American military aviation.
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