As Mr. Kjelgarrd notes, the 707 wasn't the first commercial jetliner; that honor belongs to deHavilland Comet, which entered service in 1952. But a series of disastrous crashes caused it to be temporarily withdrawn from service and redesigned, correcting the structural problem that caused some aircraft to break up in flight. But the Comet never recovered from those early setbacks, creating an opening for other manufacturers, most notably Boeing and Douglas Aircraft.
The 707 was based on Boeing's highly successful 367-80 design for the U.S. military, which became the foundation for the KC-135 tanker, the RC-135 SIGINT aircraft, and the EC-135 airborne command post, among other aircraft. First flown in 1954, the "Dash 80" airframe was easily adaptable for passenger service, allowing Boeing engineers to quickly design--and build--a commercial model. While the military version and the passenger model appeared virtually identical, there were significant structural differences between the two aircraft.
On December 20, 1957, just after 12:00 noon Pacific Time, the prototype 707 took off on its maiden flight from Boeing field. The first flight lasted only seven minutes; bad weather descended on the area shortly after the Stratoliner's takeoff, forcing Boeing to curtail the event. However, conditions quickly improved, and the 707 returned to the skies later that day, spending 71 minutes aloft on its second flight. It was an auspicious beginning.
And, Boeing's timing couldn't have been better. With the Comet program still dogged by safety concerns--and the Douglas DC-8 still in development--the Seattle-based aircraft manufacturer gained an important leg up in the commercial aviation sector. The trickle of early orders quickly became a flood.
Over the next 20 years, Boeing would build and sell almost 900 Stratoliners to airlines and air cargo firms around the world. The 707 production line actually remained open until the early 1990s, with military customers buying smaller numbers of airframes. Some of those jets entered the Air Force inventory as AWACS platforms (E-3), or long-range communications aircraft for the U.S. Navy (E-6). With military purchases, Boeing built more than 1,000 707s, over a production run that spanned nearly four decades. The manufacturer also sold hundreds of "Dash 80" models that became the foundation for other military aircraft, making that "family" of aircraft the most successful in the history of aviation.
It's also worth noting that the 707 was developed on Boeing's dime, and it's success was hardly assured when the program began in the early 1950s. With Comet's early lead--and Douglas's long history in cargo and passenger aircraft--Boeing faced a significant challenge. The company's CEO, William Allen, literally gambled Boeing's future on the 707--and his bet paid off spectacularly.
That first flight of the 707 represented a seminal moment in aviation history, marking the transition from piston to jet engine aircraft in the airline and cargo business. But development of the Stratoliner is also a tribute to corporate innovation and daring, a willing to "bet it all" on a family of aircraft that would dominate aviation for the decades that followed.
ADDENDUM: The Aviation.com article also recalls a stunning moment in development of the Dash 80, which paved the way for the 707. During an early test flight of the military jet, Boeing test pilot Tex Johnson elected to perform a barrel roll with the Dash 80. The maneuver was perfectly safe, but the sight of a four-engine transport/tanker prototype performing the stunt proved impressive. Johnson's decision earned him a tongue-lashing from William Allen, who asked him what he was doing. "Selling airplanes," Johnson replied.