Boeing B-29 Superfortress

Biggest, Fastest Highest Flying Bomber Carries Largest Bomb Load Greatest Range

By Herb Powell,
Associate Editor, Aviation

Pressurized cabin is feature of superbomber … Simplicity of plane's construction demonstrates maker's mastery of manufacture

Triumph of putting the mightiest of bombers — Boeing's great B-29 Superfortress — into the air against our enemies is, above all, the triumph of the vision, ingenuity, effort, and courage of the American aircraft industry.

With the initial bombing of Japan by this most-powerful airplane, radio and press carried word to every man-in-the-street of this huge craft's tremendous capacities — for unequaled speed, range, bomb loads, and ceiling.

But it was the unequaled capacity of our industry which made it possible. That was the primary impression taken away by those of us who have observed the production lines in the Boeing Airplane Co at Wichita and learned the detailed story of the superbomber from its makers.

Briefly, the B-29 (illustrated and also described in a table on the opposite page) is an all-metal, mid-wing monoplane with single tail and retractable dual-wheel tricycle landing gear. Power, greater than that of any other bomber, is supplied by four, l8-cyl Wright Cyclone engines of 2,200 hp each, for a total of 8,800 hp. Through reduction gears these engines turn the largest propellers in use today, four-blade Hamilton Standard airscrews of 16 ft 6 in dia. Fitted to each engine are twin exhaust-driven turbosuperchargers, a dual installation necessitated by the fact that no single supercharger available was large enough for the high horsepower engines. Armor is built into the plane's fuselage, but it does not carry a load.

A prime feature of the B-29 is its pressurized cabin. Maintained via ducts leading from the superchargers on the two inboard engines, the pressurization is held fairly static to an initial high altitude, then falls slightly at subsequent higher altitudes with the increased rarity of the outside air. Oxygen masks are supplementary at extreme heights. Supercharging on one engine will supply the pressure, hence that on the other affords a 100 per cent reserve.

Through pressurization, efficiency of the combat crew is decidedly enhanced. Pressurization, it is stated, is used in high-altitude approach to, and return from, the target where, while in action, the cabin is depressurized and demand oxygen employed. The additional comfort of the supercharged cabin in the approach flight makes the men more alert over their objective.

There are two cabin pressure regulators (by AiResearch), and again since one regulator suffices, there is a 100 percent reserve. Seals are built into the joints of the fuselage. Nose compartments are pressurized and also the 30-in. tunnel catwalk over the bomb bays and the connecting waist compartments. Aft of the latter is an unpressurized compartment. The pressure, however, is carried through this compartment by two cable conduits to serve the tail-gunner's position.

Use of pressurization brought experiments with double-pane plastic glass having an intervening air space. However, considerable distortion in depth perception through these panes resulted in a return to the single-pane. Later an improved laminated-plastic pane was devised.

Difficulties with the original blister fittings, notably a blister blow-out which sent a man out into empty space (he saved himself with his chute) brought development of the metal grille (seen in one of our photos) for reinforcement. But later a rubber blister-rim was devised which absorbed the stresses, and the grille was abandoned.

On a number of the initial planes, cabin superchargers were used. Development of the present system of pressurization with GE turbos followed, it being found that the dual superchargers could supply this added service without drain.

Weight of System Negligible

The circular section of the B-29 fuselage is best for holding pressure, as was determined earlier in the circular-section Boeing Stratoliner. Since the pressurization system automatically supplies cabin heat and also reduces the amount of oxygen that needs to be carried, the extra weight of the installation is almost negligible. There is flow control as well as pressure control in the system, and the cabin repressurizes very quickly following the depressurization phases.

Gross weight of the Superfortress is roughly, twice that of the Flying Fortress and printed reports have said the plane is capable of carrying more than 16,000 lb of bombs. Striking range of the B-29s may be judged by the fact that after flying the Western China-to-Yawata round trip in their debut bombing of Japan, most of the planes, according to the press, landed back at their bases with encouragingly big supplies of gasoline left in their tanks — enough to fly hundreds of miles (or translatable into a heavier bomb load).

Roominess of the B-29's cockpit contrasts with the tight nose accommodations of earlier bombers. There is considerable space between pilot and copilot, and on the panels facing them the former multiplicity of instruments has been minimized, a comprehensive control panel being fitted in the flight engineer's alcove immediately aft of the copilot. A step-down position in the nose greenhouse accommodates the bombardier who, by virtue of the sealed cabin's soundproofing, can converse freely with the pilots. Speaking of this soundproofing, the Superfortress is the quietest bomber we've flown in. With noise and racket obviated and shouting unnecessary, there is decidedly less crew fatigue.

In the compartment next behind the bridge are positions for the radioman (starboard) and the avigator (port). Then the 30-in-dia passageway extends rearward over the bomb bays to connect with the aft compartments for the remaining crew. Here are found the waist firing positions, followed by a bunk room sleeping four. Finally a long compartment leads to the tail turret.

True, the ship, a mass of compact installations, is complex throughout, We will provide more of the details a little later on. But our initial interest here is the mastery of manufacture it represents. Few accounts have highlighted the point which was most striking to us — that is, simplicity of fabrication.

In choosing a cylindrical-design fuselage, Boeing did much more than achieve speed through clean lines; The company thereby greatly simplified manufacture of the plane — for the 9-ft-dia sections of the body and accompanying bulkheads are initially turned out much like huge metal vats and vat covers. Built up-ended in large jigs, the sections are speedily fabricated by a number of employees working simultaneously both outside and inside.

These sections are quickly fitted together into the larger components. In final assembly there are five portions: Nose, bomb bay, bomb-bay-to-tail, three-part tail, and three-part wing. The wing center section, weighing 11 tons minus engines, is carried by overhead crane from the wing portion of the plant and lowered into place in the center section. It is noted that at Wichita, the B-29 wing is made in three parts (center section and two tips) whereas it is understood that at Renton, now in production, the wing is made in five parts in accordance with differing plant accommodations.

The whole scheme of simplified production is evidenced in the strikingly low number of man-hours required to build the ship. The exact figures cannot be given, but it can be revealed that current Superfortresses take but 1/10th the number of man-hours which were required to turn out the first model. In fact, the B-29 today is less expensive in man-hours than was the famed B-17 Flying Fortress at the same stage. Wichita is practically at the Army-set rate of production now, and an extra capacity of some 20 percent is still available.

Particularly noted was the exceptionally smooth riveting on this bomber. Virtually all rivets are driven flush, and the aluminum sheets are butt-jointed externally throughout. Likewise, the landing gear doors close exceedingly flush (drag of the Superfortress is reported to be double when the landing gear is down — additional evidence of the fine aerodynamic qualities built into the plane).

Greater-Range Airfoil

The B-29's Boeing 117 airfoil section represents further ingenuity. Specially developed for greater-range performance through refinements in drag characteristics, the resultant wing admittedly is given a high loading. But despite this, the desired high ceilings are obtained. The 117 section is strictly a Boeing development, though an NACA section is said to approximate it. The 117 is somewhat similar to the airfoil used on the Boeing Sea Ranger.

In internal structure, the wing is of web type rather than tubular-spar type as on the B-17. The web type again is simpler. Multi-cellular, it particularly gives more space for intra-wing fuel tanks. Trailing edge of the wing is approximately straight, except for a portion between inboard nacelles and fuselage. Here, the trailing edge of the huge wing flap hooks downward, decreasing aerodynamic interference between wing and body and minimizing tail buffeting during climb. Dihedral of the wing is 4½° and there is a 7° sweepback. Inboard portion of the wing employs 9/16-in Alclad, termed the thickest so far used. Aspect ratio of the wing is 11.5. Maximum wag on the wing tip in flight is 48 in as compared with 30 in on the B-17.

The exceptionally large wing flaps constitute nearly 20 percent of the wing area. In takeoff, these flaps are extended 25°. In extended position, they increase the wing area 19%. Using additional flap in landing, the B-29 "sets down" with comparable speed to the B-17. Incidentally, the Superfortress has been landed without flaps. (And it may be noted here that the plane can fly on two engines and has taken off using only three.)

Operation of the flaps is of special interest. Actuated through threaded rods, two on each wing, they roll out and down on five "I” tracks on either wing. Their control is synchronized.

The B-29 is virtually an all-electric plane, using 150 electric motors of 49 different types. Only hydraulic use is in the landing gear braking system. Included in the craft is some 11 mi of wiring. A Lawrance Aeronautical Corp 12-hp auxiliary gasoline engine gives extra power for operation of the gear in takeoff and landings.

Boeing's design of control surfaces and control elements give accent to stability and ease of handling throughout takeoff, flight, and landing. Control forces are termed even lighter than in the B-17 through refinements in design and employment of tab surfaces in the airstream to facilitate control-surface movements. The pilot "feel" is "right on the touch"; in fact, operation of the rudder is stated to require less effort than that of the B-17.

Combination servo tabs and trim tabs reduce the control force on the aerodynamically and statically balanced ailerons to a minimum. Although of different airfoil section and construction, the stabilizers and elevators are identical in plan form to those on the present B-17s. It is interesting to note that the stabilizer airfoil's leading edge turns up, giving in effect an inverted airfoil. This is to obviate stalling of the stabilizer at a critical flight attitude.

The ease with which the ship handles repeatedly impressed us when we flew in the superbomber. We observed that she taxied lightly, belying her great size. She was airborne with little more than a 2,000-ft run. And in her subsequent climbs, banks, and bursts of speed which demonstrated her fine performance, the smoothness and power seemed incongruous with her weight. We sat cross-legged in the nose, between pilot and co-pilot, as she landed, and we can report that she came in with precision. It was (if poesy will be pardoned) as though she were guided in an invisible groove curving gently to merge with the runway. With such a mammoth craft, we had expected something of a jar at impact, but none materialized. It is stated that the B-29 can land and take off in shorter space than the B-17.

To continue with the description of the plane, design of the B-29's nacelles called for incorporation of a single large air duct in the nacelle nose, thus giving a frontal oval shape. Further, sufficient space is provided in these units to accommodate the retracting landing gear and the dual superchargers while still retaining reasonable accessibility to the nacelles. The practical final design was an adjudication between the project engineers (who strove for a unit housing all needed equipment) and the aerodynamic engineers (whose purpose was minimum size and maximum streamlining for speed).

In development of the bomb bays, versatility as well as size was achieved. The initially planned bays accommodated a great weight of bombs, but only in the larger sizes. Through lengthening of the bays (and accordingly the ship itself) there is now accommodation for as great a weight in small “eggs" as in large ones.

First Dual Nose-Wheel

Use of a dual nose wheel in the B-29 landing gear — first such double unit ever devised — was necessitated by the extreme weight of the Superfortress. There were, accordingly, the problems posed by weight of the unit, also its steering and retracting. Wheels of 3-ft dia were finally decided upon, while those of the main undercarriage measure 56 in. The latter are Goodyear 16-ply Nylon SC synthetic tread tires. It is noted that the B-29's CG lies 15 ft forward of the main landing gear. Weight of the landing gear totals slightly more than 7,000 lb — roughly 6,000 lb for the main undercarriage and 1,000 lb for the nose-wheel gear.

Turning to the engines, the reduction gear employed affords an extremely low ratio, revolving the propellers only 35/100ths times as fast as the rpm of the engines. For aerodynamic reasons, it was essential to keep the speed of the prop tips subsonic, yet utilize maximum engine power. Thus, the airscrews turn unusually slow; but speed of the tips, at the periphery of the 16 ft 6 in dia, compares with the speed of tips on other planes.

Construction of Boeing's Superfortress was actually underway — in certain components — before its entire design was completed. Thus, in numerous instances portions of the airplane virtually were blueprinted after, rather than before, their construction. The B-29 is stated to have had no major "bugs"— except the "lack-of-time bug." Yet even this one was ironed out. With painstaking stick-to-it-iveness, the men behind the B-29 kept pushing the job to completion. They concentrated their efforts on whatever phases of the program could be attacked "fustest" with the "mostest" of designs, materials, and workers. The sequence didn't matter; getting the job done did.

Preliminary tunnel tests, made at the University of Washington and California Institute of Technology, won the Army order for three "X" models. But flight testing a plane which you haven't yet built is impossible. Not entirely impossible, though. Fact is that Boeing did flight test many portions of the B-29 control. systems, including all tail surfaces, before a single Superfortress was flown.

Various B-29 control surfaces, scaled down to B-17 size, were tested on a B-17C Flying Fortress. Thus the ailerons were checked. Next, scale-model tail surfaces were put through their paces on a B-17E. And tests on a scaled-down B-29 nose-wheel gear were made on a Douglas A-20 at Wright Field.

Hydraulic-Jack Static Tests

In later tests of the wings, 300,000 lb of pressure was required to prove the structures, and the former lead-shot-sacks method of static testing was impractical. Answer was a specially designed system of hydraulic jacks, which brought a great saving of labor and more accurate application of loads. Finally, according to Army stipulation, the complete airplane frame, with weight inside to simulate full equipment, bombs, fuel, crew, and ammunition load, was free-dropped from a point 27 in above the floor. The Superfortress successfully passed two such tests, one in horizontal position and one in inclined position.

Vibration in the plane has been reduced to a minimum. At first, magnesium was employed in the fabrication of the tunnel-way over the bomb bays, but because this structure tended to vibrate, there was a switch to dural.

Manufacture of the Superfortress involves basically the same principle as that used in building the B-17s — that is, precompletion of major sections on separate lines before final assembly. Because of the economy of production effort achieved in the B-29, it has been possible to employ hastily recruited, speedily trained personnel. Boeing at Seattle had started the tooling project while the bomber was still in the preliminary design stages.

A major consideration was the joining of the various sections of the plane, and therefore many man-hours were initially expended to obtain a production joint to fit together various sections of the plane with bolts, using a torsion wrench, and with no splicing of beams or skin laps required.

Master gauges were designed and manufactured to maintain interchangeability of all major Superfortress parts no matter where built. Moreover design for production is further evidenced by the clips which tie the circumferentials to the stringers, inside the fuselage skin. B-29 stringers, unlike those of the B-17, do not pass through slots cut in the circumferentials. Instead, the stringers are riveted to the skin and the circumferentials are attached to the stringers by clips.

Number of clip designs required in the plane was finally narrowed down to six, three left-hand and three right-hand. Rivet holes on the clips are prepunched, full size, as are the matching holes in the circumferentials and stringers. The clip is riveted to the circumferential as a subassembly operation. The stringer is positioned in the jig by prelocated coordinated holes, and the circumferential and clip is attached to the stringer by one Cleco pin.

In the other hole a rivet is driven. Then the Cleco pin is removed and the second rivet is driven. In effect, this is prematching holes on a large aircraft structure so that parts will fit together handily, yet very close tolerances are achieved.

In outline, there are five airframe plants in the B-29 program — Boeing at Seattle, Renton, and Wichita; Glenn L Martin at Omaha, NE; and Bell Aircraft at Marietta, GA. A major subassembly operation is handled by the Fisher Body Div of General Motors at Cleveland, Instituted was a B-29 Committee, similar in pattern to the successful Boeing-Douglas-Vega Committee coordinating production of B-l7s.

Working through various subcommittees, the B-29 Committee coordinates procurement of materials and the key phases of the work through the hundreds of subcontractors and numerous subassembly operations. Further, it prepares and maintains master production schedules, coordinates tooling requirements and distributes all design-change instructions among prime contractors. The Committee's offices are located in Seattle, where sessions are held regularly. It may be noted that there have been no subcontracting delays.

Firms Contributing

All the companies contributing to the program cannot be cited in this short space, but a few specific mentions may be made for perspective: The 2,200-hp engines are now coming from the huge Dodge-Chicago plant of the Chrysler Corp, though Wright is biggest producer; General Electric developed the fire control system which actuates the Superfortress guns; Bendix Aviation Corp supplies a super-generator; De Soto Div of Chrysler makes noses, wing edges, and engine cowling covers; Minneapolis-Honeywell produces the automatic pilot; Libbey-Owens-Ford contributes the bullet-proof glass employed; and United States Rubber Co delivers to the ship more than 200 rubber parts weighing nearly 5,000 lb. Others which may be named are A O Smith Corp (landing gear); Jack & Heintz (landing gear motors); Goodyear Aircraft (fuselage center sections and empennages); Chandler-Evans Corp (carburetors): also Briggs Mfg Co, Cessna Aircraft, Firestone Tire & Rubber Co, Hudson Motor Car Co, Maytag Co, Murray Corp of America, Ohmer Register Co, Ryan Aeronautical, and Shakespeare Products Co.

Boeing of Wichita makes some 40,000 of the total of approximately 55,000 parts (not including rivets, etc) employed in the superbomber.

Additional notes: It is stated that to the best of ability design of the B-29 Superfortress aimed at ease of maintenance of the ship and interchangeability. Means to supply the B-29s operating in the 20th Air Force have, wherever possible, followed the channels already set up. Because of the huge size of the bomber, it was necessary to devise large handling facilities for outside maintenance and repair.

The company reveals that a transport version ship, as yet undesignated, will be coming. It is stated that the new 117 airfoil and pressurized-cabin development are well fitted for such a craft. However, the specific bomber design is not considered suited for transport work.

Of leading interest in our Wichita visit was the detailed demonstration of the construction of the wing. A total of 34 extrusions are incorporated in the wing, 17 on each side. A heavy extruded duralumin chord is used for the web spar, Weighing 255 lb when machined, this chord is termed the largest employed in a production airplane.

Wing Spar Chord Practice

The following traces the right-hand front wing spar chord from the time it is received at the Boeing-Wichita warehouse until it reaches its ultimate position in the Superfortress:
Raw material is received in West Warehouse No 1, at Crib No 47.

    To Shop 120. Farnham Mills.
  1. Straighten (spar chord, 25½ ft long, supposedly is straight when received, but process here is to make sure);
  2. Mill face (process of milling down face of chord to specifications. Skin is later fastened to milled face);
  3. Mill leg;
  4. Mill taper on inside face;
  5. Mill inside face of flange;
  6. Mill warp on outside face;
  7. Saw to length (this calls for chord to be cut to exact length of 202 in);
  8. Straighten (milling and sawing reduced spar chord to approximately 217 lbs; second straightening process is to make certain no bends have resulted). Band Saw:
  9. Rough cut scarf (process of cutting out scarf, or section, where right-hand spar chord will eventually be joined with left- hand chord in center of airplane);
  10. Mill scarf;
  11. Mill side scarf, outboard (for later joining with outboard portion of inboard spar).
    To Shop 122.
  12. Router operation (sections of chord are routed out for later fitting-in of other sections of wing);
  13. Drilling operation (this is drilling of holes which later will be used for fastening on panels, or skin);
  14. Inspect.
    To Shop 420.
  15. Anodine (chord is immersed in a liquid bath to give it a protective finish);
  16. Inspect.
  1. Drill chords and webs, make joint at Station 0 (lower and upper chords are joined by means of a web, then right-hand and left-hand assemblies are placed together by means of a scarf joint at Station 0, which is center of airplane);
  2. Rivet assembly;
  3. Inspect.
    To Shop 410.
  4. Paint;
  5. Inspect.
  1. Make scarf joint at Station 202 (two outboard portions of inboard spars are scarf joined to inboard portion at this point.
    Station 510 to 510 means it is 510 in from extreme ends to Station 0, which is center of plane);
  2. Complete inboard front spar assembly.

This article was originally published in the July, 1944, issue of Aviation magazine, vol 43, no 7, pp 110-111, 278-281, 283-285.
The original article includes a leading page with 4 photos and a data table.
The photos are not credited.