{"id":1200527,"date":"2026-05-27T14:58:00","date_gmt":"2026-05-27T12:58:00","guid":{"rendered":"https:\/\/migflug.com\/jetflights\/?p=1200527"},"modified":"2026-09-04T09:32:23","modified_gmt":"2026-09-04T07:32:23","slug":"b-29-superfortress-pressurized-crawl-tunnel-engineering","status":"publish","type":"post","link":"https:\/\/migflug.com\/afterburner\/b-29-superfortress-pressurized-crawl-tunnel-engineering\/","title":{"rendered":"The B-29&#8217;s Pressurised Crawl Tunnel: 1940s Engineering Genius"},"content":{"rendered":"<p><style>.et_pb_title_container h1.entry-title { padding-top: 40px !important; }<\/style>\n<!-- mfsh:top -->\n\n<style>\n.mfsh-trigger{display:inline-flex;align-items:center;gap:10px;height:44px;padding:0 18px;margin:0 0 26px;border:2px solid #0F1720;background:#fff;color:#0F1720;font-family:inherit;font-size:14px;font-weight:700;line-height:1;letter-spacing:0;text-transform:none;cursor:pointer;box-shadow:none;transition:background .15s,color 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It cost more than the Manhattan Project. Its development consumed three billion dollars of 1944 money \u2014 call it sixty billion today \u2014 and produced an aircraft that introduced more entirely new engineering concepts to one airframe than any combat aircraft before or since. Pressurised cabins. Remote-controlled gun turrets. A computerised fire-control system. Wings so long that they generated their own structural problems. And \u2014 most curiously of all \u2014 a 34-inch-wide padded tunnel running the length of the bomb bay so the crew could crawl from the front of the aircraft to the back without dying.<\/p>\r\n<p>The tunnel was not a luxury. It was the only way to make a pressurised bomber that could also drop bombs. And in 1944, the engineers who designed it solved a problem that still seems improbable today.<\/p>\r\n\r\n<div style=\"background: #f5f7fa; padding: 18px 22px; margin: 24px 0; border-radius: 8px; border: 1px solid #e0e6ed;\">\r\n<p style=\"margin: 0 0 8px; font-weight: bold; color: #333; font-size: 15px; letter-spacing: 0.5px; text-transform: uppercase;\">Quick Facts<\/p>\r\n<table style=\"width: 100%; border-collapse: collapse; font-size: 15px; margin: 0;\">\r\n<tbody>\r\n<tr>\r\n<td style=\"padding: 6px 12px 6px 0; font-weight: 600; color: #5c91ff; white-space: nowrap;\">Aircraft<\/td>\r\n<td style=\"padding: 6px 0;\">Boeing B-29 Superfortress<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"padding: 6px 12px 6px 0; font-weight: 600; color: #5c91ff; white-space: nowrap;\">First flight<\/td>\r\n<td style=\"padding: 6px 0;\">21 September 1942<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"padding: 6px 12px 6px 0; font-weight: 600; color: #5c91ff; white-space: nowrap;\">Number built<\/td>\r\n<td style=\"padding: 6px 0;\">3,970<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"padding: 6px 12px 6px 0; font-weight: 600; color: #5c91ff; white-space: nowrap;\">Cabin pressurisation<\/td>\r\n<td style=\"padding: 6px 0;\">First combat aircraft with fully pressurised crew compartments<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"padding: 6px 12px 6px 0; font-weight: 600; color: #5c91ff; white-space: nowrap;\">Service ceiling<\/td>\r\n<td style=\"padding: 6px 0;\">31,850 ft (9,710 m)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"padding: 6px 12px 6px 0; font-weight: 600; color: #5c91ff; white-space: nowrap;\">The Tunnel<\/td>\r\n<td style=\"padding: 6px 0;\">34-inch (86 cm) diameter, ~10 m long, padded, pressurised, running OVER the bomb bays<\/td>\r\n<\/tr>\r\n<tr>\r\n<td style=\"padding: 6px 12px 6px 0; font-weight: 600; color: #5c91ff; white-space: nowrap;\">Programme cost<\/td>\r\n<td style=\"padding: 6px 0;\">~$3 billion (1944) \u2014 more than the Manhattan Project<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<\/div>\r\n\r\n<h2 style=\"padding-top: 22px;\">The problem nobody had solved<\/h2>\r\n<p>Pressurised airline cabins were a 1930s invention. The Boeing 307 Stratoliner had flown in 1938. But every pressurised airliner had something a bomber did not: a continuous, sealed fuselage with no holes in it. The B-29 needed to fly at 30,000 feet to escape Japanese fighter cover and flak, which meant the crew had to be pressurised. The B-29 also needed bomb bays that opened in flight, which meant the centre of the aircraft could not be pressurised \u2014 you cannot open a hole in the side of a balloon and expect the balloon to stay inflated.<\/p>\r\n<p>Boeing's engineers split the difference. The forward crew compartment \u2014 pilot, co-pilot, bombardier, navigator, radio operator, flight engineer \u2014 was pressurised. The rear compartment \u2014 three gunners, the central fire control gunner, and a tail gunner \u2014 was pressurised. The bomb bays in between were not. To move between the two pressurised volumes during flight at 30,000 feet, the crew used a 34-inch-diameter padded tunnel that ran OVER the bomb bays and connected the two pressurised compartments through twin pressure-tight bulkheads.<\/p>\r\n\r\n<figure class=\"wp-block-image size-large\" style=\"margin: 0 0 24px;\"><img decoding=\"async\" style=\"display: block; width: 100%!important; max-width: 100%!important; height: auto!important; border-radius: 6px;\" src=\"https:\/\/migflug.com\/afterburner\/wp-content\/uploads\/sites\/4\/2026\/05\/boeing-b-29-bomber-formation.jpg\" alt=\"B-29 Superfortress in flight\" \/>\r\n<figcaption style=\"font-size: 13px; color: #777; text-align: center; margin-top: 6px; font-style: italic;\">A B-29 Superfortress on combat operations. The pressurised cabins let crews fly at 30,000 ft above the worst flak \u2014 and the tunnel between cabins let them rest, swap positions, or relieve a wounded crewman in flight. <em>NASA \/ GRC archive<\/em><\/figcaption>\r\n<\/figure>\r\n\r\n<h2 style=\"padding-top: 22px;\">Inside the tunnel<\/h2>\r\n<p>The tunnel itself was a tube of stretched aluminium skin reinforced by hoop frames, lined inside with quilted padding and small canvas straps that crewmen used to pull themselves through. At each end the tube opened into the pressurised crew compartments through sealed bulkheads, while its skin kept the unpressurised bomb-bay air outside.<\/p>\r\n<p>Crewmen who had to make the trip during a long flight described it as awkward but tolerable. The trick was to remember that the tunnel ran above the bomb bay, which meant entering it meant climbing up and forward. The journey took less than a minute \u2014 though in an aircraft being shaken by flak, that minute could feel rather longer.<\/p>\r\n\r\n<div style=\"max-width: 550px; margin: 0 auto 28px;\"><iframe class=\"skip-lazy\" style=\"width: 100%; height: 400px; border: none; border-radius: 12px; overflow: hidden;\" src=\"https:\/\/platform.twitter.com\/embed\/Tweet.html?id=1597881932471877632&amp;theme=light\" allowfullscreen=\"allowfullscreen\" data-no-lazy=\"1\"><\/iframe><\/div>\r\n\r\n<h2 style=\"padding-top: 22px;\">Why nobody had built it before<\/h2>\r\n<p>The engineering challenge was not the concept of a tunnel. The challenge was that the tunnel had to maintain pressure differential \u2014 about 6.5 PSI between cabin pressure (set to roughly 8,000 ft equivalent) and outside ambient at 30,000 ft \u2014 while running through the structurally weakest section of the aircraft, the bomb bay roof. The aluminium skin of the tunnel had to be stiff enough to handle the pressure, light enough not to ruin the weight budget, and tight enough that thousands of rivets did not slowly bleed pressure out into the slipstream.<\/p>\r\n<p>The men engineering it had almost no precedent to work from. Boeing's chief of flight test, Edmund T. Allen \u2014 who died on 18 February 1943 when an engine fire brought down the second XB-29 prototype \u2014 was still proving the basics of the aircraft in the same period the tunnel was being refined. The tunnel was, in other words, a life-support feature engineered onto a wartime bomber with no direct precedent at all.<\/p>\r\n<h2 style=\"padding-top: 22px;\">What it made possible<\/h2>\r\n<p>The tunnel was not just for moving. It was also a sleeping platform on long missions over the Pacific. The Tokyo raids of 1944\u201345 routinely lasted fifteen hours. Crew members who were off duty would crawl into the tunnel, brace themselves against the padding, and sleep. It was, in effect, an improvised in-flight crew rest compartment, decades before such spaces became standard on long-haul aircraft.<\/p>\r\n<p>When the B-29 dropped the atomic bombs on Hiroshima and Nagasaki, the crews who carried them out relied on that same tunnel during the long flights from Tinian. It was a piece of 1941 engineering that quietly outlasted the world it was built for.<\/p>\r\n\r\n<div style=\"margin: 24px 0;\">\r\n<div style=\"position: relative; padding-bottom: 56.25%; height: 0; overflow: hidden; border-radius: 8px;\"><iframe style=\"position: absolute; top: 0; left: 0; width: 100%; height: 100%; border: 0;\" src=\"https:\/\/www.youtube.com\/embed\/lIBtpED-vUw\" allowfullscreen=\"allowfullscreen\"><\/iframe><\/div>\r\n<p style=\"font-size: 13px; color: #777; text-align: center; margin-top: 6px; font-style: italic;\">Full interior tour of a flying B-29 (\"Doc\") at the New England Air Museum \u2014 including the pressurised crawl tunnel itself.<\/p>\r\n<\/div>\r\n\r\n<p><em>Sources: Wikipedia; Pilotmall.com; nuclearcompanion.com; New England Air Museum archives; airplanes-online.com; period Boeing engineering memoranda.<\/em><\/p>\n<!-- mfsh:bottom -->\n\n<button type=\"button\" class=\"mfsh-trigger mfsh-bottom\" data-mfsh-placement=\"bottom\" aria-haspopup=\"dialog\"><svg><use href=\"#mfsh-i-share\"\/><\/svg>Share this story<\/button>\n\n<!-- \/mfsh:bottom -->\n\r\n<!-- mf-faq -->\r\n\r\n\r\n<div class=\"mf-faq-block\"><style>.mf-faq-block{margin:34px 0}.mf-faq-item:not([open]) .mf-faq-answer{display:none !important}.mf-faq-block h2.mf-faq-h{padding-top:22px;margin-bottom:14px}.mf-faq-item{border:1px solid #e2e8f5;border-radius:8px;margin:0 0 10px;background:#fff}.mf-faq-item summary{list-style:none;cursor:pointer;padding:15px 50px 15px 18px;font-weight:600;color:#1a1a1a;position:relative;line-height:1.45;user-select:none}.mf-faq-item summary::-webkit-details-marker{display:none}.mf-faq-item summary::after{content:\"+\";position:absolute;right:18px;top:50%;transform:translateY(-50%);font-size:1.5em;font-weight:400;color:#5C91FF;line-height:1}.mf-faq-item[open] summary::after{content:\"\\2013\"}.mf-faq-item[open] summary{border-bottom:1px solid #eef1f8}.mf-faq-item summary:hover{background:#f5f8ff}.mf-faq-answer{padding:14px 18px;color:#333;line-height:1.6}.mf-faq-answer p{margin:0}<\/style>\r\n<h2 class=\"mf-faq-h\">Related Questions<\/h2>\r\n<details class=\"mf-faq-item\">\r\n<summary>Why did the B-29 have a tunnel inside it?<\/summary>\r\n<div class=\"mf-faq-answer\">\r\n<p>The B-29 Superfortress had a padded crawl tunnel connecting its front and rear pressurised crew compartments over the bomb bays. Because the bomb bays themselves weren't pressurised, the tunnel let crew move between the two sealed cabins without losing pressure.<\/p>\r\n<\/div>\r\n<\/details><details class=\"mf-faq-item\">\r\n<summary>What was the B-29's crawl tunnel?<\/summary>\r\n<div class=\"mf-faq-answer\">\r\n<p>It was a roughly 10-metre-long, 34-inch (86 cm) diameter padded tube running over the bomb bays, linking the forward and aft pressurised sections of the aircraft. Crew could crawl through it to move between compartments at high altitude.<\/p>\r\n<\/div>\r\n<\/details><details class=\"mf-faq-item\">\r\n<summary>Was the B-29 pressurised?<\/summary>\r\n<div class=\"mf-faq-answer\">\r\n<p>Yes \u2014 the B-29 was the first combat aircraft with fully pressurised crew compartments, letting crews fly comfortably at high altitude without bulky oxygen gear. The bomb bays stayed unpressurised, which is why the connecting crawl tunnel was needed.<\/p>\r\n<\/div>\r\n<\/details><details class=\"mf-faq-item\">\r\n<summary>How much did the B-29 program cost?<\/summary>\r\n<div class=\"mf-faq-answer\">\r\n<p>The B-29 programme cost around $3 billion in 1944 \u2014 even more than the Manhattan Project that built the atomic bomb. It was one of the most expensive and ambitious engineering efforts of World War II.<\/p>\r\n<\/div>\r\n<\/details><details class=\"mf-faq-item\">\r\n<summary>How high could the B-29 fly?<\/summary>\r\n<div class=\"mf-faq-answer\">\r\n<p>The B-29 had a service ceiling of about 31,850 feet (9,710 m). Its pressurised compartments let crews operate at those altitudes in relative comfort, a major advance over earlier unpressurised bombers.<\/p>\r\n<\/div>\r\n<\/details><\/div>\r\n<p><script type=\"application\/ld+json\">{\"@context\":\"https:\/\/schema.org\",\"@type\":\"FAQPage\",\"mainEntity\":[{\"@type\":\"Question\",\"name\":\"Why did the B-29 have a tunnel inside it?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"The B-29 Superfortress had a padded crawl tunnel connecting its front and rear pressurised crew compartments over the bomb bays. Because the bomb bays themselves weren't pressurised, the tunnel let crew move between the two sealed cabins without losing pressure.\"}},{\"@type\":\"Question\",\"name\":\"What was the B-29's crawl tunnel?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"It was a roughly 10-metre-long, 34-inch (86 cm) diameter padded tube running over the bomb bays, linking the forward and aft pressurised sections of the aircraft. 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Its pressurised compartments let crews operate at those altitudes in relative comfort, a major advance over earlier unpressurised bombers.\"}}]}<\/script><!-- \/mf-faq -->\r\n\r\n\r\n<div style=\"background: #f0f4ff; border-left: 4px solid #5C91FF; padding: 16px 20px; margin: 32px 0 8px; border-radius: 0 8px 8px 0;\">\r\n<p style=\"margin: 0 0 8px; font-weight: 600; color: #333;\">Related Posts<\/p>\r\n<p style=\"margin: 4px 0;\">The Mosquito: Built From Trees, Faster Than a <a href=\"https:\/\/migflug.com\/aircraft\/supermarine-spitfire\">Spitfire<\/a><\/p>\r\n<\/div>\r\n","protected":false},"excerpt":{"rendered":"<p>A 34-inch-wide padded tunnel ran above the B-29&#8217;s bomb bay, letting crews crawl from front to back at 30,000 feet. It was 1944 engineering that nobody had ever attempted.<\/p>\n","protected":false},"author":27,"featured_media":1200446,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"editor_notices":[],"footnotes":""},"categories":[666,664],"tags":[],"class_list":["post-1200527","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-history-and-legends","category-military-aviation"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.5 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>B-29 Superfortress: The Pressurised Crawl Tunnel<\/title>\n<meta name=\"description\" content=\"The B-29 Superfortress cost more than the Manhattan Project \u2014 and hid a padded crawl tunnel linking its pressurised crew cabins over the bomb bay.\" \/>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" 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