They were flown by numerous airlines, although United was Douglas' primary customer. We saw them in several episodes. The ones that stick in my mind are when McGarrett flew to Los Angeles in "Once Upon a Time" (see below) and when the crooked professors flew in with their stolen travelers' cheques in "3000 Crooked Miles to Honolulu." The grand old lady originally flew with Pratt & Whitney JT4D engines.
Today, it seems that only five of Donald Douglas' DC-8s are still flying. One is owned by Samaritan's Purse, a charitable organization that ferries supplies to areas in need. It uses what appear to be Rolls Royce engines, yet it still looks just as sleek and sharp as ever. See it in this video: Samaritans Purse DC-8 - YouTube. As an extra bonus, a USAF C-17 is seen in the background as the DC-8 slows on the runway before turning off.
I'm partial to Douglas aircraft. They just keep on flying. Here's a partial list:
* DC-3 (1935) is still in service with Buffalo Airways in the Northwest Territories of Canada, as well as with other airlines flying to remote locations. Her tough landing gear and never-ceasing Pratt & Whitney R1830 engines make it possible to touch down in any terrain, from ice to rain forests. It can carry loads of up to 7,000 pounds.
* DC-4 (1942) first served United Airlines. Today, it is in service with Buffalo Airways. It is capable of carrying loads of up to 20,000 pounds.
* DC-8 (1958) the second successful fanjet passenger airliner. It first served with United and, today, is in service with several air freight operators.
* DC-9 (1965) the first successful regional jet largely served with Delta and Northwest Airlines. She continues to serve smaller airlines and remains beloved by all who have flown aboard her, if only for her distinctive-sounding Pratt & Whitney JT8D engines, her swept empennage, and her soaring t-tail. The aircrafts known as MD-80 through MD-95 are DC-9 extensions, properly numbered DC-9-80 through DC-9-95. With the MD prefix (McDonnell Douglas, given after McDonnell and Douglas merged in 1967), she is affectionately known as Mad Dogs.
Several have served in the US military as the C-47 (DC-3), C-54 (DC-4), and C-9 Nightingale (DC-9).
* After serving with distinction in World War II, the DC-3/C-47 was used in part during the Korean War to transport injured servicemen from the field to hospitals. The quick transportation is credited with markedly reducing in-transit deaths. When the Lockheed C-141 StarLifter entered service in 1965, in-transit deaths were even more greatly reduced due to the faster speeds and the pressurized cabin, both of which were especially beneficial to servicemen with chest wounds.
* The DC-3/C-54 served in World War II and is especially remembered for its participation in the Berlin Airlift.
* The DC-9/C-9 served in Vietnam as a hospital aircraft. It also served as an executive aircraft for the military.