By the late 1940s two developments encouraged Boeing to begin considering building a passenger jet.
The first  was the maiden flight in 1947 of the B-47 Stratojet.
The second was the maiden flight in 1949 of the world's first jet airliner, the de Havilland Comet.
Boeing President Bill Allen led a company delegation to the UK in summer 1950, where they saw the Comet fly at the Farnborough Airshow, and also visited the de Havilland factory at Hatfield, Hertfordshire where the Comets were being built.
Boeing felt it had mastered the swept wing and podded engines which it saw as key technologies that would enable it to improve on the Comet.

In 1950 Boeing tentatively produced a specification for a jet airliner dubbed the Model 473-60C. The airlines were unconvinced because they had no experience with jet transports and were enjoying success with piston engined aircraft such as the Douglas DC-4, DC-6, Boeing Stratocruiser and Lockheed Constellation.

Boeing was experienced at selling to the military but had not enjoyed the same success with civil airliners.
This market was dominated by Douglas which was adept at meeting the needs of airlines by refining and developing its range of propeller-driven aircraft, and in 1950 was marketing the forthcoming DC-7.
Boeing decided the only way to overcome the airlines' suspicion of the jet – and of itself – was to show them a completed aircraft.

As the first of a new generation of passenger jets, Boeing wanted the aircraft's model number to emphasize the difference from its previous propeller-driven aircraft which bore 300-series numbers.
The 400-, 500- and 600-series were already used by missiles and other products, so Boeing decided that the jets would bear 700-series numbers, and the first would be the 707.

Boeing had studied developments of its existing Model 367 (the KC-97 Stratofreighter) incorporating swept wings and podded engines; and chose to build the 367-80, which retained little of the KC-97 except the upper fuselage diameter (and the possibility of building some of the fuselage with existing tooling).
Although the design was announced publicly as the Model 707, the prototype was referred to within Boeing simply as the Dash 80 or "-80".

The Dash 80 fuselage was wide enough at 132 inches (335 cm) for five-abreast seating; two on one side of the aisle and three on the other.
The fuselage diameter for the production KC-135 was widened to 144 inches (366 cm) and Boeing originally hoped to build the 707 fuselage with that width.
By the time the Boeing company committed to production, the decision had been made to design the production model 707 as a six-abreast design, with a larger 148-inch-diameter (376 cm) fuselage, after C. R. Smith, CEO of American Airlines, told Boeing he wouldn't buy the 707 unless it was an inch wider than the then-proposed Douglas DC-8 passenger jet.
This decision did not unduly delay the introduction of the production model since the -80 had been largely hand-built, using little production tooling.
By the late 1940s two developments encouraged Boeing to begin considering building a passenger jet. The first  was the maiden flight in 1947 of the B-47 Stratojet. The second was the maiden flight in 1949 of the world's first jet airliner, the de Havilland Comet. Boeing President Bill Allen led a company delegation to the UK in summer 1950, where they saw the Comet fly at the Farnborough Airshow, and also visited the de Havilland factory at Hatfield, Hertfordshire where the Comets were being built. Boeing felt it had mastered the swept wing and podded engines which it saw as key technologies that would enable it to improve on the Comet.
In 1950 Boeing tentatively produced a specification for a jet airliner dubbed the Model 473-60C. The airlines were unconvinced because they had no experience with jet transports and were enjoying success with piston engined aircraft such as the Douglas DC-4, DC-6, Boeing Stratocruiser and Lockheed Constellation.
Boeing was experienced at selling to the military but had not enjoyed the same success with civil airliners. This market was dominated by Douglas which was adept at meeting the needs of airlines by refining and developing its range of propeller-driven aircraft, and in 1950 was marketing the forthcoming DC-7. Boeing decided the only way to overcome the airlines' suspicion of the jet – and of itself – was to show them a completed aircraft.
As the first of a new generation of passenger jets, Boeing wanted the aircraft's model number to emphasize the difference from its previous propeller-driven aircraft which bore 300-series numbers. The 400-, 500- and 600-series were already used by missiles and other products, so Boeing decided that the jets would bear 700-series numbers, and the first would be the 707.
Boeing had studied developments of its existing Model 367 (the KC-97 Stratofreighter) incorporating swept wings and podded engines; and chose to build the 367-80, which retained little of the KC-97 except the upper fuselage diameter (and the possibility of building some of the fuselage with existing tooling). Although the design was announced publicly as the Model 707, the prototype was referred to within Boeing simply as the Dash 80 or "-80".
The Dash 80 fuselage was wide enough at 132 inches (335 cm) for five-abreast seating; two on one side of the aisle and three on the other. The fuselage diameter for the production KC-135 was widened to 144 inches (366 cm) and Boeing originally hoped to build the 707 fuselage with that width. By the time the Boeing company committed to production, the decision had been made to design the production model 707 as a six-abreast design, with a larger 148-inch-diameter (376 cm) fuselage, after C. R. Smith, CEO of American Airlines, told Boeing he wouldn't buy the 707 unless it was an inch wider than the then-proposed Douglas DC-8 passenger jet. This decision did not unduly delay the introduction of the production model since the -80 had been largely hand-built, using little production tooling.
Boeing 367-80