Boeing B-47B rocket-assisted take off on April 15, 1954. The Boeing B-47 Stratojet jet bomber was a medium-range and -size bomber capable of flying at high subsonic speeds and primarily designed for penetrating the Soviet Union.

A major innovation in post-World War II combat jet design, it helped lead to the development of modern jet airliners. While the B-47 never saw major combat use, it was the mainstay of U.S. Air Force Strategic Air Command in the 1950s.

The B-47 arose from a 1943 U.S. Army Air Forces requirement for a jet bomber and reconnaissance aircraft that could reach Nazi Germany in the event that Great Britain fell.

The next year, the requirement evolved into a formal request for a bomber with a specified speed of 500 mph (800 km/h) or more, a range of 3,500 mi (5,600 km), and a service ceiling of 40,000 ft (12,200 m).

It envisioned using the General Electric TG-180 turbojet engine, then in development. By this time, the war in Europe was obviously winding to a close. General Henry H.

“Hap” Arnold, head of the USAAF, asked the prestigious expatriate Hungarian aerodynamicist Theodore von Kármán, of the California Institute of Technology, to form up a committee of American scientists to go to Europe and examine captured German technology.

The result was the “Scientific Advisory Group”. One of the members was Boeing’s chief aerodynamicist, George Schairer.

During his visit to Germany, Schairer examined data obtained by German aircraft manufacturers on the advantages of swept wings, and became so convinced of the merits of such a design that in May, 1945 he wrote a letter to Boeing management suggesting the matter be investigated.

Preliminary designs

North American, Convair, and Boeing submitted proposals. The first Boeing proposal, the Model 424, was a modification of a conventional propeller-driven bomber design, basically a scaled-down version of the Boeing B-29 fitted with four jet engines.

Meanwhile, the USAAF had awarded study contracts to all three aircraft manufacturers working on the jet bomber project, as well as to Martin, which had also decided to join the competition.

All of the competing bombers, including the North American B-45, Convair XB-46 and Martin XB-48 would have conventional straight wings with four to six engines, and would lack the performance of the swept wing B-47.

NACA design tests

The National Advisory Committee for Aeronautics (NACA, the ancestor of NASA) performed wind tunnel tests on a composite model of the designs submitted by the manufacturers. (The three submissions were generally similar.)

The NACA wind tunnel tests showed that the Boeing model suffered from excessive drag. Boeing engineers then tried a revised design, the Model 432, with the four engines buried in the forward fuselage, but although it had some structural advantages there was little effect on drag.

At this point Boeing engineers turned to the German swept-wing data. A little design work by Boeing aerodynamicist Vic Ganzer led to an optimum sweepback of 35 degrees.

Boeing modified the Model 432 design with a swept wings and tail, resulting in the Model 448, which was presented to the USAAF in September 1946. The Model 448 retained the four TG-180 engines in the forward fuselage and, at the instigation of project manager George Martin, added two more TG-180s buried in the rear fuselage to provide greater range and performance.

Boeing submitted the Model 448 to the USAAF, only to have it rejected immediately. The Air Force strongly disliked fitting the engines in the fuselage, since that made engine fire or disintegration catastrophic. The engines would have to be moved back out on the wings.

That led straight back to the drag problem, but the engineering team came up with a clean, elegant solution, with the engines in streamlined pods attached to the wings.

This innovation led to the next iteration, the Model 450, which featured two TG-180s in a single pod mounted on a pylon about a third of the way outboard on each wing, plus another engine slung from the wingtip.

The Air Force liked the new configuration, and so the Boeing team continued to refine it. One problem was landing gear. There was no space for landing gear in the thin wings, and trying to put conventional tricycle landing gear in the fuselage would have ruined the aircraft’s streamlining and degraded its performance.

Furthermore, the USAAF was now also insisting that the bomber be able to carry an atomic bomb. As such weapons were very big at the time, that meant a long bomb bay, further limiting space for landing gear.

The solution was a “bicycle” landing gear configuration, with the two main gear assemblies arranged in a tandem, not a side by side, configuration. Outrigger landing gear was to be fitted to the inboard engine pods. The concept had already been tested on a modified Martin B-26 Marauder aircraft.

However, bicycle landing gear made it difficult for a pilot to “rotate” an aircraft into a nose-up position for takeoff. Again, the solution was simple: the landing gear was designed so that the nose-up position was the default.

This little change would have a very pleasing effect on an aircraft that was already shaping up to be very elegant, giving the machine the appearance of being ready to leap into the air even when it was sitting still.

There were some other tweaks to the design, such as a wingtip extension to improve range. This had the effect of moving the outboard engines from a wingtip position to an underwing position towards the end of the wings.

USAAF selects Boeing

The USAAF was very pleased with the refined Model 450 design, and in April 1947, the service ordered two prototypes, to be designated “XB-47″. Assembly began in June 1947. People involved with the project were very excited, since they believed (correctly as it turned out) they were working on a breakthrough in aircraft design.

The XB-47 prototype first flew on 17 December 1947, with test pilots Robert Robbins and Scott Osler at the controls. The aircraft flew from Boeing Field in Seattle to the Moses Lake Airfield in central Washington, in a flight that lasted 52 minutes.

There were no major problems, except that Robbins had to pull up the flaps with the emergency hydraulic system and the engine fire warning lights kept popping on, the sensor technology being very unreliable at the time. Robbins reported that the flight characteristics of the aircraft were good.

Design stages
XB-47
Description
The XB-47 looked unlike any contemporary bomber, described by some observers as a “sleek, beautiful outcome that was highly advanced”. The 35-degree swept wings were shoulder-mounted, with the twin inboard turbojet engines mounted in very neat pods, and the outboard engines tacked under the wings short of the wingtips.

With the exception of a change from the shoulder-mounted wing configuration to being under the fuselage, most future airliners would use a similar configuration, with the engines mounted in under-wing pylons.

The airfoil was 11 times as wide as it was thick. This unusual thinness (dry, no fuel tanks) was believed to be necessary to attain high speed (.86 Mach), but the wing’s flexibility was a concern. It could flex as much as five ft (1.5 m) up or down, and major effort was expended to ensure that flight control could be maintained as the wing moved up and down.

As it turned out, most of the worries proved unfounded. (Wing “twist” limited tree-top speed to 425 knots to avoid control reversal) The wings were fitted with a set of Fowler flaps that extended well behind the wing, to enhance lift at slow speeds.

The bicycle landing gear dictated by the thin wing consisted of a pair of large wheels fore and aft of the bomb bay, with small outrigger wheels carried on the inboard twin-jet pods.

Performance and engines

The performance of the Model 450 design was projected to be so good that the bomber would be as fast as fighters then on the drawing board, and so the only defensive armament was to be a tail turret with two .50-cal Browning machine guns, which would in principle be directed by an automatic fire-control system.

The two XB-47s were not fitted with the tail turret as they were engineering and flight test aircraft, and in fact the prototypes were not fitted with any combat equipment at all.

Fuel capacity was an enormous 17,000 U.S. gal (64,400 liters), compared to 5,000 U.S. gal (19,000 L) on the B-29. That meant that maintaining fuel trim to ensure a stable center of gravity in flight would be very critical co-pilot duty.

The first prototypes were fitted with General Electric J35 turbojets, the production version of the TG-180, with 3,970 lbf (17.7 kN) of thrust. Early jet engines did not develop good thrust at low speeds, so to assist in takeoffs in heavily loaded condition, the XB-47 prototype had provisions for fitting 18 solid-fuel rocket-assisted takeoff (RATO) rockets with 1,000 lbf (4.4 kN) thrust each.

Fittings for nine such units were built into each side of the rear fuselage, arranged in three rows of three bottles.

Drag chutes

A related problem was that the aircraft’s engines would have to be throttled down on landing approach. Since it could take as long as 20 seconds to throttle them back up to full power, the big bomber could not easily do a “touch and go” momentary landing.

A small “approach” chute was provided for “drag” so that the aircraft could be flown at approach speeds with the engines throttled at ready-to-spool-up medium power. Typical was an hour of dragging this chute around the landing pattern for multiple practice landings.

The aircraft was so aerodynamically slick that rapid descent (“penetration”) from high cruise altitude to the landing pattern required dragging the deployed rear landing gear.

Unusually heavy wing loading (weight/wing area) required a high (180 knot) landing speed. To shorten the landing roll Air Force test pilot Major Guy Townsend promoted the addition of a 32 ft (9.75 m) German-designed “ribbon” drag chute. (Jet engine thrust reversers were still a far-future concept.)

Crew and loads

The XB-47 was designed to carry a crew of three in a pressurized forward compartment: a pilot and copilot in a long fighter-style bubble canopy, and a navigator in a compartment in the nose. The copilot doubled as tail gunner, and the navigator as bombardier. The bubble canopy could pitch up and slide backward, but as the cockpit was high off the ground, crew entrance was through a door and ladder on the underside of the nose.

Total bombload capacity was to be 10,000 pounds (4.5 tonnes). Production aircraft were to be equipped with state-of-the-art electronics for navigation, bombing, countermeasures, and turret fire control.

A final change in the B-47E was that most of the windows in the nose were deleted, with only one left on each side. However, many pictures of B-47Es show them with the full set of windows used on the B-47B. Whether the number of windows varied through B-47E production, or whether these were B-47Bs updated to B-47E specification, is unclear.

The B-47E-II featured only minor changes from late production B-47E-Is. The B-47E-III featured an ECM suite, consisting of a radar jammer in a bulge under the fuselage plus a chaff dispenser, as well as improved electrical alternators.

The B-47E-IV was a much more substantial update, featuring stronger landing gear, airframe reinforcement, greater fuel capacity, and a bombload uprated to 25,000 pounds (11,300 kg), though the bomb bay was once again shortened because of the introduction of more compact nuclear weapons.

Another improvement was the introduction of the MA-7A BNS, a major step up from its predecessors. The MA-7A included the AN/APS-64 radar, with a range as long as 240 miles (390 km). The AN/APS-64 could be used as a long range “identification friend or foe (IFF) transponder” interrogator to allow a B-47E-IV to find a tanker or other B-47, or it could be used as a high-resolution ground-targeting radar.

The B-47E-IV retained the optical bombsight, though this was rarely used.
A total of 1,341 B-47Es were produced. 691 were built by Boeing, 386 were built by Lockheed, and 264 were built by Douglas. Most B-47Bs were rebuilt up to B-47E standards. They were given the designation of B-47B-II, though it appears that in practice they were simply called B-47Es.

Early years

When B-47s began to be delivered to the Air Force, most crews were excited about getting their hands on the hot new bomber. The bomber was so fast that in the early days, the B-47 set records everywhere it flew without even trying.

The aircraft handled well and comfortably in flight, with a fighter-like light touch to the controls. The big bubble canopy enhanced the fighter-like feel of the big aircraft with improved all around vision, but the inherent design would cause variations in internal temperatures for the 3-man crew.

However, it took the Air Force until 1953 to turn the B-47 into an operational aircraft. The big aircraft was sluggish on takeoff and too fast on landings, a very unpleasant combination of circumstances. Furthermore, if the pilot put the machine down at the wrong angle on the bicycle landing gear, the aircraft would “porpoise”, bouncing fore-and-aft.

If the pilot didn’t lift off for another go-round, instability would quickly cause the bomber to skid onto one wing and cartwheel to its destruction. Because the wings and surfaces were flexible and bent in flight, low altitude speed restrictions were necessary to ensure that basic flight control surfaces remained effective.

Improved training led to a good safety record, and few crews felt the aircraft was inherently unsafe or too demanding, but apparently there were aircrews who had little affection for or were even afraid of the B-47. Crew workload was also high, with only three crew members to keep the B-47 flying right.

The B-52 Stratofortress, in contrast, generally had six crew, with much less cramped accommodations. While the original XB-52 used a fighter-style canopy, production versions used a conventional flight deck.

Training and problems

The B-47′s reliability and serviceability were also regarded as good. The only major problem was that the avionics were not very reliable, unsurprising given the vacuum tube technology available at the time, and the need to locate some equipment outside the pressurized crew compartment.

Much work was done to improve the reliability of the avionics, but they remained something of a maintenance headache all through the B-47′s operational life.

Several models of the B-47 starting in 1950 included a fuel tank inerting system, in which dry ice was sublimated into carbon dioxide vapor while the fuel pumps operated or while the in-flight refueling system was in use.

The carbon dioxide was then pumped into the fuel tanks and the rest of the fuel system, ensuring that the amount of oxygen in the fuel system was low, and thereby reducing the probability of an explosion in flight. Ten carbon dioxide tanks and heaters were involved. The system was implemented largely to reduce risks from static electricity discharges occurring during in-flight refueling.

Prime years

By 1956 the U.S. Air Force had 28 wings of B-47 bombers and five wings of RB-47 reconnaissance aircraft.

The bombers were the first line of America’s strategic nuclear deterrent, often operating at forward bases in the UK, Morocco, Spain, Alaska, and Guam. B-47 bombers were often set up on “one-third” alert, with a third of the operational aircraft available sitting on the runway, loaded with fuel and nuclear weapons, crews on standby, ready to take off for no-holds-barred attack against the USSR at short notice.

Crews were also trained to perform “minimum interval takeoffs (MITO)”, with one bomber following the other into the air at intervals of as little as 15 seconds, to get all the bombers on the way as fast as possible. MITO could be very hazardous, as the bombers left turbulence and, with water injection, dense black smoke that blinded pilots in the following aircraft.

B-47 bombers apparently performed training missions in which they penetrated Soviet airspace in numbers. The facts behind these missions remain controversial, with some claiming that Curtis LeMay ordered them without presidential knowledge or approval.

The B-47 would be the backbone of SAC into 1959, when the B-52 began to take over and the B-47 wings started to be cut back. Actual B-47 production had ceased in 1957, though modifications and rebuilds continued after that.

Operational practice for B-47 bomber operations during this time went from high altitude bombing to low altitude strike, which was judged more likely to penetrate Soviet defenses. Bomber crews were trained in “pop-up” attacks, coming in at low level (425 knots) and then climbing abruptly on nearing the target before releasing a nuclear weapon, and the similar “toss bombing” procedure, in which the aircraft released the weapon while climbing and then rolled away to depart the target area before the bomb fell back down and detonated.

Later years

Stress and fatigue incurred in low altitude operations led to a number of wing failures and fatal crashes, and an extensive refit program was initiated in 1958 to strengthen the wing mountings. The program was known as “Milk Bottle”, named after the big connecting pins that were replaced in the wing roots.

A B-47 and a F-86 Sabre collided in 1958. The F-86 crashed and the B-47 losing one of its single jet engines catching fire after just leaving Homestead Air Force Base. The pilot had to “safe” soft drop a Mark 15 thermonuclear weapon off the coast of Savannah, Georgia (where it remains) and safely returned to base.

The only B-47s to see anything that resembled combat were the reconnaissance variants. They operated from almost every airfield that gave them access to the USSR, and they often probed Soviet airspace, and on occasion, B-47 pilots were caught in situations from which mostly speed and evasion in retreat saved them. At least five of these aircraft were fired on, and three of these were shot down.

The B-47s fired back with their tail turrets, though it is uncertain if they scored any kills, but in any case these were the only shots fired in anger by any B-47. These missions became impractical upon the introduction by the Russians of the trans-sonic MiG-19. (Similar to the performance of USAF F-100)

Final phaseout of B-47 bomber wings began in 1963, and the last bombers were out of service by 1965. The very last USAF operational aircraft was grounded in 1969. The U.S. Navy kept specialized test aircraft in occasional use up to 1976.

The final recorded flight of a B-47 was on 17 June 1986, when a B-47E was flown from the Naval Air Weapons Station China Lake, California, to Castle Air Force Base, California, to be put in the air museum there. There are at least 15 B-47s that survive on static display, but none are still flying.

Specifications (B-47E)
Data from Quest for Performance
General characteristics
Crew: 3
Length: 107 ft 1 in (32.6 m)
Wingspan: 116 ft 0 in (35.4 m)
Height: 28 ft 0 in (8.5 m)
Wing area: 1,428 ft² (132.7 m²)
Airfoil: NACA 64A(0.225)12 mod root and tip
Empty weight: 79,074 lb (35,867 kg)
Loaded weight: 133,030 lb (60,340 kg)
Max takeoff weight: 230,000 lb (100,000 kg)
Powerplant: 6× General Electric J47-GE-25 turbojets, 7,200 lbf (32 kN) each
* Zero-lift drag coefficient: 0.0148 (estimated)
Drag area: 21.13 ft² (1.96 m²)
Aspect ratio: 9.42
Performance
Maximum speed: 607 mph (527 kn, 977 km/h)
Cruise speed: 557 mph (484 kn, 896 km/h)
Combat radius: 1,749 NM (2,013 mi, 3,240 km) with 20,000 lb (9,000 kg) bombload
Ferry range: 4,037 NM (4,647 mi, 6,494 km)
Service ceiling 33,100 ft (10,100 m)
Rate of climb: 4,660 ft/min (23.7 m/s)
Wing loading: 93.16 lb/ft² (454.8 kg/m²)
Thrust/weight: 0.22
Lift-to-drag ratio: 20.0 (estimated)
Armament
Guns: 2× 20 mm M24A1 cannons
Bombs: 25,000 lb (11,000 kg) of ordnance, including:
2× nuclear bombs, or
28× 500 lb (230 kg) conventional bombs