Boeing introduced its first jet airliner, the Boeing 707, in 1958 with Pan American World Airways. Though not the first commercial jetliner ever produced – the de Havilland DH106 Comet first flew in 1949 – the 707 is often credited as a pioneer of the Jet Age in the 1960s.
A legendary jetliner
Over the 21 years that the Boeing 707 was in production, several variants were manufactured: the main ones being the -120, -320, and -420. Across these, some factors remained constant, such as the 12-foot-14-inch (3.759-meter) fuselage width and 42,000-foot (12,800-meter) service ceiling.
Other aspects, such as passenger capacity, aircraft length, and range were improved and increased over time.
The original: 707-120/-120B
The Boeing 707-120 was the first standard model of the 707, powered by Pratt and Whitney JT3C turbojet engines. After its inaugural flight on December 20, 1957, the -120 received certification from the Federal Aviation Authority (FAA) on September 19, 1958.
In 1959, Boeing replaced the JT3C engines with more efficient JT3D variants. With these new engines, a “B” suffix was added to the aircraft name, renamed to the 707-120B. The first of these entered service in 1961 with American Airlines.
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Having the shortest wingspan of all 707 variants, at 130 feet and 10 inches (39.88 meters), the -120 was also the lightest, standing at an operating empty weight of 127,500 lb (57,600 kg). It had a Maximum Take-Off Weight (MTOW) of 257,340 lb (117,00 kg).
Due to its smaller size compared to other variants, the Boeing 707-120 required a take-off distance of only 7,500 feet (2,300 meters), and had a range of 3,600 nm (6,700 km). This aircraft type could accommodate 174 passengers in one class or 137 in two classes. A total of 300 Boeing 707-120/-120Bs were produced.
New and improved: 707-320/-420
As with many aircraft types, the first variant is never the last. Boeing sought to improve the -120 by increasing its capacity and upgrading its engines with higher-powered JT4As. On January 11, 1958, a new variant of the 707 was introduced: the Boeing 707-320.
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189 passengers could fly on the aircraft in a single-class configuration, and 141 in two classes. While the fuselage width across all variants remained the same, the -320 had an extended length, at close to 153 feet (46.61 meters), compared to the -120, which was just slightly over 145 feet (44.22 meters).
This variant also had a stretched wing, spanning 142 feet and 5 inches (12.85 meters), which could carry more fuel. Due to its increased size, the -320 had an operating empty weight that was over 15,000 lb (7000 kg) heavier than its predecessor. Its increased fuel capacity also meant that the -320 was designed with a larger MTOW – 312,000 lb (141,700 kg).
These changes in the -320 called for a longer take-off distance of 10,700 feet (3,250 meters), and the aircraft could reach a range of 3,750 nm (6,940 km) thanks to its larger fuel capacity. With this variant, Boeing 707s were now capable of flying from Europe (westbound) to the West Coast of the US, and from the US to Japan.
At this point, Boeing also introduced the 707-420, which were essentially -320s fitted with Rolls-Royce Conway 508 turbofans. The first airline to carry passengers on the -420 was Lufthansa, in 1960. 106 units of -320/-420 aircraft were produced.
Small change, big range: 707-320B
Two years after the original variant first took to the skies, the -320B was introduced. This improved version had the same length and passenger capacity as the earlier -320, but was fitted with curved low-drag wingtips, instead of blunt wingtips.
This modification increased wingspan by more than 3 feet (1 meter). The -320B also had a heavier operating empty weight of 148,800 lb (67,500 kg) and a 21,600 lb (9,800 kg) increase in MTOW.
As a result of its aerodynamic refinements, the -320B required a take-off distance that was 700 feet (250 meters) less than that of the initial -320. Its landing distance (5,900 feet / 1,800 meters) was also lesser than what was required by both the -120 (6,500 feet / 2,000 meters) and -320 (7,200 feet / 2,200 meters).
The most significant improvement in the -320B was its range, which was an impressive 1,250 nm (2,360 km) more than the -320. The 707 could now reach up to a distance of 5000 nm (9,300 km). Boeing built 174 units of the -320B.
The final variant: 707-320C
The Boeing 707-320C version saw the -320 aircraft having a convertible passenger-to-freight configuration (the “C” in its name meant “cargo”). Boasting a new cargo door and strengthened floor, this variant also allowed for an increased number of passengers: 195 in one class and 141 in two.
Wing improvements were incorporated into the -320C model, which were later added to the -320B from 1963 onwards. These were known as the Boeing 707-320B Advanced.
While only a few -320Cs were delivered as pure freighters – most were passenger aircraft – airlines took a liking to this variant. This versatile aircraft type became the most popular 707 variant because operators saw potential in its second-hand value, resulting from its ability to be converted into a freighter. 377 units were produced, the most of any 707 variants.
A limit to improvements
As the demand for air travel exponentially grew, facilitated by the success of the Boeing 707, larger aircraft were needed to keep up with demand. Unfortunately, Boeing’s original jetliner could not be enlarged any further.
If Boeing was to continue increasing the capacity of its 707, more powerful engines which required more ground clearance would be needed. For this to be possible, a new landing gear was required. All in all, it would take a drastic redesign rather than just minor improvements to the aircraft.
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Commercial production of the Boeing 707 ceased in 1978 – 1,010 units had been manufactured by then. Though the production line continued assembling military versions of the 707 until 1991, commercial operators started looking to the Boeing 747 to meet demand.
As the Jumbo Jet’s popularity skyrocketed, the era of the Boeing 707 inevitably came to an end.