Key to the entire system was Price’s cabin pressure regulator. “It consisted of an inlet valve, which regulated the flow of ventilating air to the cabin, and an outflow value to control the discharge of air,” says Lavelle. In between, a maze of valves, venturis (short tubes with constricted throats), and sylphons (pleated metal bellows) interacted with one another, maintaining conditions inside the airplane as outside pressure varied. Incoming air was drawn via rows of slits on the wings’ leading edges, heated, and then pumped into the cabin; the 307 “exhaled” near its tail. It all happened largely automatically; reporters were so impressed they called the regulator a kind of “brain.” Boeing patent attorney Charles Reynolds saw a bright future for the device. “In a few years,” he told Price, “this ought to be in every plane in the country.”
But getting it to work was another matter. “Once engineers completed the working prototype, they felt the best approach would be to use an altitude chamber to replicate high-altitude pressures, Lavelle says. “But their request was denied, so they used an empty 50-gallon oil drum to fabricate a test rig.” By “supercharging” it with compressed air from a blower, Price and chief engineer Jim Cooper could make the pressure in the drum correspond to different “altitudes,” imitating what the 307 would experience in flight. To their delight, the new cabin regulator held these pressures perfectly—strong proof their concept was sound.
Still, nothing could replace the next step: actual flight tests. “Engineers would have to climb on board the aircraft with their parachutes, climb into the lower fuselage accessory compartment, and make adjustments on the regulators at altitudes of up to 20,000 feet,” Lavelle says. “Fleece-lined jackets—it was cold!” recalls Cooper. With so many parts, the regulator was somewhat delicate. “One failure was traced to human fingerprints left inside the bellows,” says Lavelle. “The oily residue was skewing altitude measurements by 2,000 feet.” Once perfected, production of the regulator was licensed to Garrett AiResearch of Los Angeles.
Aircraft of that day sometimes leaked when it rained, so making the 307 reasonably airtight (even after double rows of rivets) required some unorthodox quality control. All the skin joints were covered by “paw tape,” a Dupont neoprene product developed for the XC-35, recalls Bob Dickson, then newly hired on Boeing’s assembly line. “Then you drove the rivets through. To test for leaks, you covered the joints with Ivory soap and [after pumping in air] watched for bubbles,” he says. It worked. For a final test, a dozen workers soaped down each pressurized cabin, carwash-style, then watched for bubbles and listened for whistling air. (Engineers still leak-test airliners today, but they use much neater ultrasound instruments.) A test rig stressed each of the 307’s curved, plexiglass window panes to nearly triple their expected flight pressure. Cockpit windows also were coated with an alloy to block ultraviolet rays, as flying above the clouds risks sunburn. “After all,” pronounced legendary test pilot Eddie Allen in Boeing’s official account of the 307, “the great thing about pressurizing, is that you [shouldn’t] know you’re up there.” By late 1938, he’d taken the 307 to 22,000 feet.
Despite those successes, TWA major stockholder John Hertz, the rent-a-car king, canceled TWA’s order for five 307s, saying that at $315,000 each, the new airplanes were too expensive. Desperate, Jack Frye contacted Howard Hughes. The young billionaire, who was planning to fly around the world, liked the idea of an over-weather craft. In classic fashion, Hughes bought controlling interest in TWA.
“Hughes spent a lot of time poking around the plant,” Dickson recalls, “frankly, often looking like he’d slept in his car.” Perhaps seeking more comfortable digs, Hughes purchased one extra airliner, for himself.
Tragedy struck in March 1939, when the 307 prototype crashed, killing all 10 aboard (seven Boeing employees, two executives from Royal Dutch Airlines, which was considering buying a 307, and a representative from TWA). Wing and tail modifications solved the airplane’s technical shortcomings, and Claire Egtvedt dubbed his new airplane the Stratoliner, to highlight what he called “travel in a new region of the atmosphere.” “Outside scientific circles,” he told reporters with suitable flourish, “[this] substratosphere has been regarded as something mystical. But before many years, I believe we will all become familiar with this superhighway of the air.”
Travelers agreed. “See you at breakfast,” New York Times aviation writer August Loeb imagined telling friends in Los Angeles as a Stratoliner took off at night from New York’s LaGuardia Field. Loeb praised industrial designer Raymond Loewy’s lavish interiors (“with dressing room for men, and ‘charm room’ for ladies”) and listened in as stewardesses explained the virtues of supercharged cabin air to passengers. What was “Stratolining” like? By night, cloudless skies resembled a planetarium; by day, vistas from Meteor Crater to the Grand Canyon were “visible in a single glance,” wrote Loeb’s colleague at the Times, Malcolm Kerr. Although it wasn’t truly stratospheric (which begins 37,000 feet up), to the pre-World War II crowd, it must have seemed like space tourism. And with a flight from New York to California (with two scheduled stops) taking just 14 hours and nine minutes, Loeb wrote, “the dream of overnight air travel across the United States, has come true.”
Because pressurization enabled it to attain an altitude of 20,000 feet, the Stratoliner offered a vastly smoother ride than long-haul competitors like the Douglas DC-3. Publicity stills of Stratoliner passengers relaxing while their children read the comics offered a new image of flight. Travelers now were living, eating, even shaving in the air. “There is no sensation of altitude, no gasping for air, no shivering in cold drafts,” went one review in Popular Science in 1940. As wide as the later 707, the Stratoliner had room for a crew of five and 33 passengers.
Though pricey, with a cross-country ticket costing $1,000 in 1940 (the equivalent of about $15,000 today), “Stratoliners were always booked,” says Mike Lavelle. For a while, everything went “Strato”—fans could don a Stratoliner hat (guaranteed weatherproof), dress in Stratoliner blue (like the seat color), or join Stratoliner clubs (for tall people).