On February 18, 1943, with World War II raging in Europe and Asia, a hulking structure rolled onto the tarmac of Boeing Field, about five miles south of Seattle’s city center. Veteran Boeing test pilot Edmond T. “Eddie” Allen, joined by 10 technicians and engineers, was chosen for the test flight.

They stood aboard the XB-29, one of only two experimental prototypes of an all-new bomber designed to punish the Axis powers. It was the most technologically advanced aircraft in the world, and its existence was strictly classified. But joining so many different envelope-pushing technologies to a single airframe came with some serious risk, and it was a risk that would unfold in horror after only 20 minutes into the flight.

Allen radioed the tower, indicating an engine fire had started and he’d have to return immediately to land. The first blaze was quickly extinguished, but then a second started, and soon, the hulking giant was losing altitude too quickly to manage. Two crew members leapt from the burning plane as it only barely managed to avoid colliding with Seattle’s downtown skyscrapers. Their chutes didn’t open in time to save either of their lives.

The bomber eventually collided with Seattle’s Frye and Company meatpacking plant, killing all on board as well as 19 more inside the building. The aircraft, and the nature of the crash, were made classified. While newspapers did report on the crash, none ran any photos or discussed the type of aircraft involved.

“The B-29 was a groundbreaking program, and like any such program, you want to try to hide the design and development stage as much as possible from your adversaries,” Shawn M. Bohannon, command historian for the U.S. Air Force Global Strike Command tells Popular Mechanics. “Even once the aircraft was rolled out, there was still a level of secrecy involved with its different components.”

The crash was a massive setback for a program that could not afford to fail. But despite the tragic test flight and ongoing production struggles—blame resource shortages and an unskilled workforce—President Franklin D. Roosevelt still pledged to base at least 175 B-29s in China, within striking distance of Japan, in a year’s time. The U.S. government had already placed an order for 1,500 of the new bombers, but when Roosevelt made that promise, fewer than 15 were airworthy.

The race for the bomber of the future was on, and its legacy would be lasting.

“Quite simply, the B-29 was the gateway aircraft to the modern United States Air Force,” Bohannon says. “It laid the groundwork for all future strategic aircraft in the U.S. Air Force inventory.”

The Battle of Kansas

The front lines of World War II were in Europe and the Pacific, but Kansas was its own kind of battleground as engineers and assembly workers fought against terrible weather and repeated delays in production of the B-29.

With development and production happening more or less simultaneously, technical problems plagued the effort. By some estimates, each of the major features introduced on the B-29 would normally have seen up to five years of testing under normal circumstances. Instead, factory workers without any aviation experience were tasked with assembling the massive bomber and modifying parts they didn’t understand in order to piece together the disparate new technologies.

The B-29 would be the first pressurized and soundproofed bomber cabin, allowing the crew to fly comfortably and communicate without the need for large headsets.

“They were incredibly innovative in how they decided to pressurize the aircraft, which led to incredible crew comfort,” says Bohannon. While comfort may not sound like something you really need in a bomber, it can have a direct effect on crew capability and overall wartime strategy, he says.

“Look at how incredible crew fatigue could be on a B-17 bomber, say, in Europe,” Bohannon says. “You’re exposed to the elements at above 20,000 feet and you’re having to be on oxygen bottles and wearing heavy winter flight gear. It’s not only about how nerve-wracking it can be in combat, but just the fatigue these crews experienced. You didn’t have that in the B-29, and that, in itself, lent to the range of the bomber.”

The bomber was designed to be an answer to the Army Air Corps’ request for a “hemisphere defense weapon,” meaning it needed exceptional range. The B-29 had been initially intended for the European front, but it soon became clear that it would be fighting mostly in the Pacific, making range an increasingly essential part of its design. It also had to be able to manage the massive weight of burgeoning new weapons systems, all while offering precision strike capability from high altitudes.

At the time, only one engine could handle the job: the 2,200-horsepower Wright R-3350. But prior to the purchase of the engine for the B-29, Curtiss-Wright had slowed development of it in favor of other, better-selling power plants. That meant the engines were installed in B-29 airframes without sufficient testing.

In order to meet the seemingly impossible demand for new bombers, a massive hiring and training enterprise—coupled with efforts aimed at simplifying production—revolutionized aircraft manufacturing. Using a workforce that had largely never even touched an airplane at the time, Boeing managed to shrink the per-bomber build time from 150,000 hours to just 20,000. By the end of the war, Boeing’s plant in Wichita would produce 4.2 B-29 Superfortresses per day.

Nonetheless, most bombers that rolled off the assembly line weren’t ready for combat, and had to fly directly to over-taxed modification centers where space was so limited that many of the planes were modified and repaired outdoors in the punishing Kansas winter.

“I was appalled,” Gen. Henry H. “Hap” Arnold, the aviation pioneer and general officer tasked with meeting the country’s bomber needs, recalled. “There were shortages in all kinds and classes of equipment. The engines were not fitted with the latest gadgets; the planes were not ready to go.”

Ultimately, despite the gargantuan effort, setbacks made Roosevelt’s 175-plane promise impossible. But by May 1944, 130 B-29s had made the 11,500-mile voyage to their new air strips in India and China. Ready or not, the B-29 was headed to war.

Even though he didn’t quite meet his production goal, Arnold’s successes were significant, and before the end of the year, President Roosevelt would promote Arnold to five-star general, making him the only such general in all of Air Force history.

Right Tech, Wrong Strategy

The B-29 was undoubtedly the most advanced bomber ever built at the time, with innovations like a longer, more narrow wing equipped with large flaps that allowed the Superfortress to cruise at high altitude without compromising its ability to manage lower speeds during takeoff and landing.

The pressurized and temperature-controlled fuselage left many aircrews feeling like they were flying the “Cadillac” of aircraft, aviators would say over the years, and Boeing’s five remote-controlled weapons systems meant gunners could engage enemy fighters while tucked away inside the aircraft, rather than in exposed gun positions. The turrets included a pair of .50-caliber machine guns above and behind the cockpit, an aft turret near the vertical tail, and two more beneath the fuselage.

In an astonishing technological feat for the 1940s, the gunners could aim using computerized sights, and each of them could control any of the aircraft’s turrets from their seated positions inside the fuselage.

The B-29 also came equipped with two radar systems. The first was either an AN/APQ-13 or AN/APQ-7 Eagle radar system (depending on mission set) that was used to aid in precision targeting. Each of these systems were accurate enough to allow crews to put bombs on target through cloud cover too thick to see through, which was a significant improvement over line-of-sight bombing operations in older aircraft. A second AN/APG-15B airborne radar gun sighting system was added to help the gunners accurately target enemy fighters as well.

In June 1944, less than two years after the first XB-29 ever took to the skies, the U.S. Army Air Force took the bomber into combat for the first time. But the showing was poor, partly because no one had taken the strong winds that can be found at various altitudes (now known as jet streams) into account.

The mission called for 100 B-29s to depart airstrips in India and fly toward targets in Bangkok, Thailand, but only 80 of them successfully made the journey. Once they were over their targets, the 80 bombers produced only meager results.

“The B-29 was designed from the outset with an almost 32,000-foot service ceiling, so it was designed to be a high-altitude bomber,” Bohannon says. “Then they found out that it was not particularly effective at those high altitudes over Japan, due to generally poor weather and, of course, the jet streams that nobody knew about yet at that time. Winds of over 150 to 200 miles per hour would make high-altitude precision bombing accuracy impossible.”

Soon, Marines fighting a brutal island-hopping campaign in the Pacific had captured air strips in places like Guam and the Mariana Islands chain, placing B-29s within striking distance of the southern tip of mainland Japan. But once again, the high-altitude precision bombing strikes weren’t nearly as effective as intended. Even worse, the bombers were struggling with continued technical issues, brought about by keeping such a massive bomber aloft at such high altitudes.

“Sometimes we had enemy fighters over, around, and under us, shooting at us,” Wallace Van Eaton, a pilot that flew 23 combat missions as a B-29 co-pilot, recounted from the cockpit of his restored bomber in 2017. “I saw a lot of planes shot down. We just kept going and had to put our trust in the Lord … and I’m still here.”

It soon became clear that the advanced bombing computers weren’t quite advanced enough for high-altitude runs. Because of cloud cover, bomber crews could rarely see their targets visually, forcing them to rely on their radar and computer systems that weren’t capable of compensating for the powerful high-altitude winds.

The Superfortress had more technology packed into its fuselage than any aircraft before it, but the strategy failed to capitalize on the bomber’s strengths, and it was beginning to make the $3 billion effort seem like a failure.

Coming Down From the Clouds

Major General Curtis E. LeMay had made a name for himself as the commander of the 3rd Bombardment Division, fighting in the European theater with the B-29’s predecessor, the B-17. By July 1945, with victory over Nazi Germany all but assured, the general would try to turn things around for the B-29 bombing efforts in the Pacific.

Upon taking over the 20th Bomber Command, LeMay took a look at the high-altitude precision strike playbook, and promptly discarded it as a poor way to leverage the new B-29.

LeMay went on to develop new bomber tactics that would take the B-29s out of the clouds and send them screaming in over Japanese cities at low levels, relying on incendiary bombs to destroy Japan’s industrial complex.

“He effectively took the B-29, which was designed to be a high-altitude bomber, and scrapped that entire idea in favor of low-level bombing at 5,000 to 7000 feet. And it proved to be an incredibly effective low-level aircraft even though it was never designed with that in mind,” Bohannon says.

After setbacks and disappointments riddling the design, production, and even bombing operations of the B-29, the plane finally found its footing using LeMay’s new low-level tactics. Concerns that the massively expensive bomber was a failure all but vanished as the aircraft proceeded to deliver firebombing raids to Japan.

In a raid on March 9, 1945, 334 B-29 bombers took off from new airstrips in the Mariana Islands bound for Tokyo and, in the span of just a few hours, dropped 1,667 tons of napalm-filled incendiary bombs on the Japanese capital. That raid remains the single deadliest bombing operation ever.

But the B-29’s most infamous moment would arrive on August 6, 1945. From the cockpit of the “Enola Gay,” a B-29 named after the pilot’s mother, Paul Tibbets dropped the first ever atomic bomb used in combat operations over Hiroshima.

“The nose lurched up—I mean it lurched dramatically—because if you immediately let 10,000 pounds out of the front, the nose has got to fly up,” Tibbets told Bob Greene on NPR’s Morning Edition during an August 2000 interview. “We made our turn, we leveled out, and at the time that that happened I saw the sky in front of me light up brilliantly with all kinds of colors.”

“At the same time,” Tibbets continued, “I felt the taste of lead in my mouth. And where we had seen the city on the way in, I (now) saw nothing but a bunch of boiling debris with fire and smoke and all of that kind of stuff. It was devastating to take a look at it.”

The 10,000-pound weapon was unlike anything the world had seen before, killing an estimated 70,000 to 100,000 people in a single blast. A few days later, on August 9, another atom bomb would be dropped from the bay of another B-29.

Soon thereafter, the Japanese surrendered, bringing World War II to a close.

Gateway to Modernity

The B-29 Superfortress is the product of a strange transition point in aviation science, bridging the divide between the open-air early bombers and the rapid advancements of the forthcoming Cold War. B-29 bombers continued to fly through the Korean war, where they were beginning to show their age compared to new jet-powered platforms, and the mighty Superfortress was ultimately retired only 16 years after it was introduced.

But that short operational window led to a number of variations on the platform that would help usher in technologies that would go on to serve as the very basis of American air superiority in the decades to come.

“The B-29 was a proven airframe, and it’s remarkable how many variants were ultimately spawned from it,” Bohannon says. “Perhaps the most famous, of course, were the silver-plated B-29s used for atomic missions that served as the post-World War II strategic deterrent force.”

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Those silver-plated bombers weren’t the only glimpses of the future the Superfortress would offer. The XB-29G carried experimental jet engines in its bomb bay for testing during flight. Other variants included the F-13, which was a B-29 converted for aerial reconnaissance, and the SB-29 “Super Dumbo,” which carried a droppable lifeboat under its belly for air-sea rescues.

Three B-29s, forced to land in Soviet territory after a bombing raid on Japan, were captured and reversed engineered to develop their Tupolev Tu-4 bombers. The Soviet Union would go on to build 847 of them. Although the B-29 was retired in 1960, China continued to fly Soviet-built Tu-4s until as late as 1988.

But the most significant technological leap came in the form of the KB-29 tanker, which was a bomber converted specifically for carrying fuel to resupply aircraft mid-flight—now the backbone of America’s current airborne operations.

A Fading Memory

The B-29 was a whirlwind platform, rushed from its first day of design to its final day in combat, which makes its legacy as the gateway to the modern Air Force that much more incredible.

“The aircraft might have been superseded in the 1960s as a bomber, but it proved to have very long legs after all,” says Bohannon.

Today, only 22 B-29s remain, scattered across museums and airfields, and only two—named FIFI and Doc—are still flightworthy.

In the modern world, the F-35 Joint Strike Fighter has yet to reach full-scale production despite having been flying for 13 years. The B-29, the bomber that ended World War II, went from drawing board to combat in under four years.

A feat that will likely never be seen again in aviation history.