<p>During World War II the urgent need to concentrate on combat aircraft inevitably slowed the development of transport aircraft. Boeing designed a transport version of the B-29 Superfortress in 1941-42, and received a US Army Air Force order for three XC-97 prototypes in December of the latter year, but was unable to complete one until mid-1944. Farsighted Boeing could see that this might put the firm at a competitive disadvantage after the war, because although Lockheed's Constellation and Douglas's DC-4 and DC-6 were smaller and basically older, they would be immediately available.
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<p>In 1942, however, Boeing had to shelve thoughts of the airline market and concentrate on production for the USAAF. By 20 July of that year it completed an engineering summary for the Model 367, and this was soon accepted. Changes from the bomber were mainly confined to the fuselage. While keeping the lower part of the B-29 fuselage (with cargo/baggage holds instead of bomb bays) the Model 367 added a much larger new upper lobe with a diameter of 11 ft (3.35 m), which introduced what became known as the double-bubble cross-section seen on most of today's airliners (though today the sides are filled in to reduce drag). At the time the fuselage of the first XC-97 looked enormously large, and gave a whale-like appearance with a bluff bow that belied the aircraft's extremely high speed.
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<p>The first prototype, which received the type name Stratofreighter, made its maiden flight at Seattle on 15 November 1944. Handling was as good as that of a B-29, and speed and mission capability were more than amply demonstrated on 9 January 1945 when the same ship, AAF no. 43-27470, flew from Seattle to Washington DC with a 20,000-lb (9,072-kg) payload in 6 hours 4 minutes at the remarkable average speed of 383 mph (616 km/h). By this time the 2,200-hp (1,641-kW) Wright R-3350-23 engines had been replaced by R-3350 57As rated at 2,325 hp (1,734 kW), but Boeing had plans for a much more powerful version, paralleling the XB-44 (later B-50) bomber. The USAAF ordered six YC-97s broadly similar to the three prototypes, as well as four of the more powerful type: three YC-97As and a YC-97B.
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<p>The YC-97As introduced the 28-cylinder Pratt & Whitney R-4360 Wasp Major engine, initially rated at 3,000 hp (2,237 kW), which in fact fitted inside a slimmer cowling than the R-3350. The nacelles were of different outline, and new four-bladed propellers were fitted with broad square-tipped paddle blades formed by welding together thin steel sheets. To meet the need for holding a straight course with an outer engine stopped, the vertical tail was made much taller, with provision for folding, and other changes included the addition of complete thermal de-icing and a switch to the new 75ST alloy for the structure of the wings and certain other parts. A total of 5,867 Imp gal (26,670 liters) of fuel was accommodated in a new fuel system with 35 nylon bladder cells, three in the center-section and 16 in each outer wing. There was provision for 41,000 lb (18,598 kg) of cargo which could be loaded via a giant rear ramp door and positioned by an electric hoist on rails running the full 78 ft (23.8 m) length of the hold. Trucks or small armored vehicles could drive on board, and in the troop carrier role no fewer than 134 seats could be installed, or 83 litters (stretchers) and four medical attendant seats. The single YC-97B (45-59596) was valuable as the prototype of what Boeing hoped would be a commercial version. A VIP personnel transport, it had rows of circular windows, upper-deck seating arranged 3 + 2 for 80, an upper-deck galley, and (a feature that was to be a memorable attraction of the civil machine) a spiral stairway down to a lower-deck lounge. The rear ramp door was removed, but in addition to a full passenger load the YC-97B could carry 17,000 lb (7,711 kg) of cargo and baggage in lower-deck holds.
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<p>The first of the six YC-97s, last of the Wright-engined Boeings, flew on 11 March 1947. These machines transformed the Air Transport Command (later MATS, Military Air Transport Service) route to Hawaii. In 1948 the first of the YC-97As flew at an average daily utilization of between nine and 12 hours on the Berlin Airlift, greatly relieving fears that the Wasp Major and its twin new-design General Electric turbochargers would prove unreliable. Boeing had few worries over the viability of the C-97, and in 1948 the newly-created USAF ordered 27, soon increased to 50. These first production Stratofreighters had Hamilton Standard propellers, and were visibly distinguished by a chin excrescence housing APS-42 weather radar.
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<p>Outbreak of war in Korea in June 1950 opened the funding floodgates, and brought orders for C-97s far surpassing Boeing's wildest dreams. These came on top of gigantic orders for the B-47 and B-52 jet bombers, and Boeing was hard-pressed to find enough floor space and manpower. The entire C-97 and Stratocruiser program was eventually moved to the Renton plant, while Seattle and Wichita were assigned other tasks. By far the biggest requirement was for inflight-refueling tankers for SAC, largely in support of Boeing's own B-47, and this led to the tests with 'flying boom' refueling on three KC-97As, followed by the production KC-97E, KC-97F and KC-97G. The massive run of 592 of the last brought the total up to 888 military C-97s in all, the final one going out of the door on 18 July 1956.
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<p>In typical Boeing style, the aircraft was developed out of all recognition. Internal fuel capacity was originally a little over 7,000 US gal (26,498 liters), but from the KC-97E tanker the addition of a 7,200-US gal (27,258-liter) group of tanks on the main floor in the fuselage brought the total up to a remarkable 14,990 US gal (56,750 liters), almost all of which could be transferred through the patented Boeing flying boom worked by an operator laying in the rear fuselage. All mainstream tankers were designed for rapid conversion to transports, with removal of the fuselage tanks, boom and operator station. The final model, the KC-97G, transferred part of the fuel to giant fixed underwing tanks, leaving enough room in the fuselage for a large cargo payload without any modification.
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<p>The vast SAC tanker force, 20 aircraft to every bomber wing, gradually became all-jet as the Boeing KC-135A replaced the KC-97 Stratotanker from 1957. Many of the well-liked KC-97s soldiered on as pure transports (C-97G), but as late as 1964 a considerable number serving with the Air National Guard, some of them in Vietnam, were fitted with the jet booster pods taken from KB-50s to give extra height and speed, thus becoming KC-97Ls. The only other C-97s with anything like the same performance were the two YC-97Js fitted with the 5,700-hp (4,250-kW) Pratt & Whitney T34-5 turboprops. Other variants included the MC-97C casualty evacuation aircraft, VC-97 flying command posts, EC-97 electronic reconnaissance platofrm and HC-97 search and rescue aircraft.
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<hi style=hdr1>Specification</hi>
<hi style=hdr2>Boeing—KC-97G Stratofreighter</hi>
<list style=aflst>
<item><hi format=bold>Type:</hi> inflight refueling tanker and transport
<item><hi format=bold>Powerplant:</hi> four Pratt & Whitney R-4360-59 radial piston engines, each rated at 3,500 hp
<item><hi format=bold>Performance:</hi> maximum speed 375 mph; service ceiling 35,000 ft; range 4,300 miles
<item><hi format=bold>Weights:</hi> empty 82,500 lb; maximum take-off 175,000 lb
<item><hi format=bold>Dimensions:</hi> wing span 141 ft 3 in; length 110 ft 4 in; height 38 ft 3 in; wing area 1,720 sq ft
