The DC-8 performed a dive at altitude to achieve the feat.

On June 5, 1969, the Soviet-built Tupolev Tu-144 became the first passenger jet to fly supersonic, beating the Anglo-French Concorde by four months. While this is most people's recollection of how supersonic passenger travel began, that's not entirely accurate. Eight years before either the Tupolev Tu-144 or the Concorde broke the sound barrier, a Douglas DC-8 had already done it.

Douglas began working on the DC-8 project in the 1950s after the United States Air Force said it was looking to purchase 800 jet tanker aircraft. Thinking that they had time and that the Air Force would, as they had done in the past, place orders for planes with more than one manufacturer, Douglas was shocked when Boeing got the entire contract.

Boeing was further down the road with developing the Boeing 707, and when the Air Force asked for tankers, the KC-35 was born. It also didn't hurt that the Seattle planemaker had a ready-to-go boom air-to-air refueling system that it had developed for the KC-97 Stratofreighter.

Shocked by the Air Forces' decision, Douglas complained to Washington, but the complaints fell on deaf ears. Now committed to the project, Douglas pushed ahead with developing the DC-8 as a passenger plane.

At the time, aircraft like the Vickers Viscount turboprop were replacing piston-driven engines leading Douglas to believe that airlines could be reluctant to purchase passenger jets. The huge financial outlay needed and the technical challenges involved in maintaining the planes gave airlines cold feet. However, Douglas knew that if it could get one major airline to invest in passengers jets, the others would have to follow.

Douglas did not have to wait long, with Pan American World Airways placing an order for 25 Douglas DC-8s and 20 Boeing 707s in October 1955. On hearing that Pan Am would be introducing the four-engined jets, other airlines worldwide began to follow suit.

To get the DC-8 FAA certified as soon as possible, Douglas used ten planes for testing flights. On August 21, 1961, while performing a test flight to collect data on a new leading-edge design for the wing, a test pilot flew the DC-8 through the sound barrier.

The aircraft, later sold to Canadian Pacific Air Lines, was crewed by pilot William Magruder, copilot Paul Patten, flight engineer Joseph Tomich, and flight test engineer Richard H. Edwards. Taking off from Edwards Air Force Base in California, the passenger jet was accompanied by a chase aircraft piloted by Chuck Yeager. In 1947 Yeager became the first pilot to break the first pilot to fly supersonic when he broke the sound barrier while flying a Bell X1.

Now flying in an F-104 Starfighter, Yeager accompanied the DC-8 to an altitude of 52,000 feet. From there, Magruder put it in a half-a-G pushover. As the aircraft dived at around 45,000 feet, it passed Mach 1.0 for about 16 seconds before recovering to level flight.

When speaking about the flight and the planning that went into it with the Smithsonian Magazine flight test engineer Richard H. Edwards said:

"They had to determine the pushover load factor, the dive angle, to be sure they got to Mach 1.01 at a rather high altitude, so the airspeed wouldn't be that high up there."

The speed of sound at altitude is not 700 miles per hour: it's a lot less because of the lower temperatures.

"The aerodynamics department, I think under Roger Shaufele, prepared a set of charts. The Mach number itself isn't used in a dive as a target because it's much more accurate to use airspeed. So every thousand feet I would read off to Bill the airspeed at the next altitude. As we were coming down, I was talking almost all the time."

When asked what it felt like to fly through the sound barrier and if there was any sensation, Edwards said:

"Not at Mach 1.0, but at .96, the plane buffeted for a while. It went away a little above .96, and it came back as we slowed down to .96."

Following the successful test flight, Douglas President Jackson McGowen met the crew for lunch, and Vice President of Flight Testing John Londelius gave them each a $1,000 bonus.

Journalist - Mark is an experienced travel journalist having published work in the industry for more than seven years. His enthusiasm for aviation news and wealth of experience lends itself to some excellent insight, with his work cited in Forbes amongst other publications. Based in Alicante, Spain.