Y1B-17 (YB-17)

First, a bit of information about U.S. (Army) Air force nomenclature - 'X' preceding a type name designates a prototype, and 'Y' a service test aircraft.  As the B-299 was a privately funded venture by Boeing, it never officially received the 'X' prefix. However it was still referred to as the XB-17 in official literature!

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With similar bureaucratic logic, what should have been the 'YB-17' was officially named the Y1B-17 due to the special source of funding used to purchase them.  As ever, the sinews of war are money - and the bean counters always have their way.

 

Nevertheless, the first production model YB-17 rolled off Boeing's assembly line on September 30, 1936, representing a major milestone. After final assembly, the sleek new bomber was carefully trucked to the company's hangars at Boeing Field for extensive ground testing before taking to the skies.

 

The 13 Y1B-17s received the serial numbers 36-149 through 36-161 in the Air Corps' numerical sequencing system. While not the finalized production variant, these aircraft allowed the Air Corps to thoroughly evaluate and test the new Boeing bomber design in an operational environment before committing to mass production.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Y1B-17 port side, showing the same svelte lines as the B-299 (Model 299)

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Within Boeing's model numbering system, the Y1B-17 production test aircraft was directly derived from the pioneering Model 299 prototype. However, one major difference distinguished the Y1B-17 from its forebear - the choice of powerplants.

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The Pratt & Whitney R-1690 Hornet radials used on the Model 299 had undergone incremental upgrades over the years. But their single-row design was nearing the limits of potential power increases. In contrast, Wright's R-1820 Cyclone engine, though still a 9-cylinder radial, featured a longer stroke design that offered higher output and more room for future development.

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Both Boeing's engineers and the Air Corps personnel at Wright Field agreed - the Cyclone represented a better long-term engine fit for the advanced Y1B-17 bombers. So when the initial production batch of 13 aircraft was delivered, they came equipped with the R-1820-39 version of the Cyclone. This uprated the bomber's power over the prototypes, with the R-1820-39s producing 930 horsepower each for takeoff. Cruising speed increased to 175 mph, while top speed reached 239 mph. Service ceiling climbed to 27,000 feet and the range extended to 2,430 miles when cruising at 14,000 feet.

 

By switching to the more potent and versatile Wright Cyclones, the Y1B-17 ensured it had propulsion that could grow with the aircraft's needs. This decision reflected Boeing's commitment to fielding a true strategic bomber capable of taking on ever-increasing roles and missions as the program evolved.

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The Y1B-17 also incorporated several notable changes and refinements based on testing and feedback.

One area that saw revisions was the wing fuel tankage layout, likely to optimize capacity and balance. Additionally, the leading edges of surfaces like wings and tail were fitted with rubber deicing boots to prevent formation of performance-sapping ice buildup.

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Two exterior features helped distinguish the Y1B-17 from its predecessor: prominent carburettor air scoops protruding from the top of each engine nacelle, and redesigned gun blisters or "cupolas." The new cupolas had a more open, streamlined framing to provide better visibility for the defensive gunners manning the machine guns.

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Front three-quarter view of a Y1B-17 showing clearer glazing on the nose turret as well as a view of the carburettor air scoops clearly visible on top of the engine nacelles

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Speaking of armament, the Y1B-17 retained the same potent gun layout as earlier - one 30-caliber Browning in the nose and belly positions, with 50-caliber Brownings comprising the remaining firing points. However, the fields of fire and maneuverability of these powerful but enclosed weapon stations drew some criticism during testing.

 

In response, engineers at Wright Field conducted experimental work exploring improvements to the gunnery systems and sighting arrangements. Maximizing the defensive firepower of the "Flying Fortress" remained a top priority as the program progressed.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Y1B-17 from the front - clearly showing two of its four Wright Cyclones with the carburettor intakes visible on top of the engine nacelles as well as the new single oleo landing gear struts

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A notable difference between the Y1B-17 production models and the original Model 299 prototype could be found in the design of the main landing gear.

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On the prototype, the main undercarriage utilized a hoop-type leg design with two oleos (shock struts) per main wheel position. While sturdy, this setup made tire changes extremely cumbersome. To swap a wheel, the entire axle had to be dropped, as the hoop encircled and trapped the wheel assembly.

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To simplify maintenance, Boeing redesigned the Y1B-17's main landing gear to instead use a single oleo-type shock strut for each main wheel position. This eliminated the hoop constraint, allowing wheels to be removed and remounted to the stub axles much more easily.

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However, one aspect remained the same - the main wheels still retracted forward and upwards into the inner engine nacelles during flight, with the lower half of the wheel well remaining open and exposed in the traditional manner of that era.

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So while streamlining the ability to service the tires, the Y1B-17's gear maintained the prototype's overall retraction scheme and aerodynamic design. This balanced the needs of simplicity and quick maintenance turnarounds with proven low-drag flight characteristics.

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As the aircraft development progressed, Boeing engineers continued modifying the design, including adding two supplemental fuel tanks for extended range. With each iteration, the designation inched towards the eventual Y1B-17 models destined for service testing with the Air Corps.

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The first test flights of the new Y1B-17s did not go entirely smoothly. On December 7, 1936, Captain Stanley Umstead from Wright Field's flight test unit took the aircraft with serial number 36-149 aloft. However, trouble was brewing even before leaving the ground.

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36-149 was fitted with an experimental bi-metallic disc brake system that proved problematic. Despite warnings from a Boeing engineer aboard, Umstead overworked the brakes severely during taxi, causing them to overheat catastrophically. Once aloft, an engine malfunction then forced Umstead to abort the test mission and bring the bomber back for an emergency landing.

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What Umstead didn't realize was that the brakes had been rendered virtually useless - the extreme temperatures had welded and fused the brake plates together. As 36-149 touched down, the wheels simply froze up instead of rolling. The $300,000 aircraft slammed nose-first into the runway at Boeing Field, skidding to a stop in an inglorious pile of crumpled aluminum.

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Y1B-17 36-149 after nosing over

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The highly public mishap was an inauspicious debut for the Y1B-17 test planes. It highlighted the challenges and perils that Boeing's crews faced while working out the kinks on such an advanced, heavy aircraft design. But the show had to go on - the Y1B-17 program would recover and these issues would soon be resolved through further testing on the road to operational service.

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Though repairs on the damaged 36-149 were completed swiftly, allowing it to resume test flights by January 2, 1937, the mishap had inflicted another injury - this time to Boeing's hopes for a major production order.

Critics pounced on the high-profile accident as evidence that the Y1B-17 was simply too large and unwieldy for its pilots to handle safely. Its sheer size and complexity made it unforgiving of even small miscalculations. Detractors argued that the future belonged to smaller, nimbler aircraft rather than such costly "four-engined behemoths."

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Suddenly, the pioneering Y1B-17's bulk and capabilities - the very qualities meant to make it a strategic bombing force to be reckoned with - were being turned against it. Boeing had to regain the confidence of skeptics who felt the aircraft maker was blindly pursuing an airplane that was just too much for its own good.

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The accident revealed more than just technological growing pains. It exposed a fundamental philosophical debate raging within the Air Corps over the shape of the modern bomber force. Should the future be staked on super-sized, long-range aircraft like the Y1B-17? Or was the massive, quarter-million-dollar bomber an example of misguided ambition?

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As the Y1B-17 program pressed on, Boeing faced an uphill battle to prove their aircraft was no more a lumbering giant, but the versatile, hard-hitting vanguard of a new strategic air attack capability.