The DC-7C was, I understand, designed by Douglas for the specific remit of being able to cross the Atlantic, non-stop, going east to west-the "Holy Grail" of air transport in the 1950s. They achieved this through, firstly, a very clever time- and money-saving piece of design. More fuel capacity was needed, so Douglas spliced five foot long extra sections of wing into the wing roots, thereby providing a lot more fuel tankage and incidentally giving the aircraft more lift, to cope with the increased weight from the extra fuel and extra passengers-see below.
This had the added benefit of taking the engines further outboard from the cabin, making for less engine noise, which was a particularly important consideration for very long flights, of the type described by Peter47. They also stretched the fuselage by 42 inches, to increase passenger load.
More powerful engines and four-bladed propellors were added, as well as a taller, more squared off tail fin (than those on the DC-6, DC-7 and DC-7B), the latter to cope with the extra power. Douglas also placed saddle tanks on top of each engine, to give yet more fuel capacity.
The end result was a very beautiful (in my opinion), streamlined and distinctive airliner, representing the peak of piston-engined airliner design, along with Lockheed's Starliner.
Douglas's clever time- and money-saving design also allowed them to get the DC-7C into service more than a year ahead of the Starliner. Lockheed, for its ultimate piston-engined airliner, went for the much more expensive and time-consuming option of designing an entirely new laminar flow wing, presumably to give the extra fuel tankage, tho I've never quite understood why they went to the (extra?) trouble of making it a laminar flow wing-can anyone explain that? The result was that the DC-7C achieved far more sales than the Starliner (121 as against 43).
I have also read that, on night flights, long purple exhaust flames were visible, coming out of the engines, across the wings and saddle tanks, full of highly flammable Avgas! Perfectly safe, but I think some passengers would have needed some reassurance from the crew.
It was such a shame that, because of the advent of and tremendous impact of the entry into service of the 707 and DC-8 in the late fifties, the DC-7C and Starliner had such a short reign at the top, from 1956 to 1960.
A magnificent aircraft and I was privileged enough to see one or two in the air and on the ground in the early 1970s, tho its glory days were long over by then. Then, after a thirty year gap, bar a couple of wrecks at Stansted in c1980, I saw (because I went out of my way to get to see them) quite a no of DC-7Cs, from 2002 to 2005: two at Cordoba, Spain, one of which was subsequently (and very sadly) scrapped, but the other is now, I understand, on display, in pretty decent condition, in Cordoba town centre. These were fully converted fire bombers, acquired by their final owner, Basaer, who, I understand, went bust in 1999, from T and G Aviation in the States.
Then two on Gran Canaria, both ex-Spantax, one of which has subsequently been scrapped. The other still survives, but is in pretty poor condition. Then one used by Geneva airport as a fire trainer-again subsequently scrapped. Then one at Fairbanks, last used by Brooks Fuel there-don't know what will happen to it following their shutting down last year.
Apart from that, there is one at the Musee de l'air at Le Bourget, not, apparently, in very good condition and very difficult to get to see, because it is in their "Reserve stock." And one at Goodyear, Arizona, apparently in pretty good condition and owned by "Pyramid Oil," tho I have heard that the actual owners are a religious cult who plan to escape the end of the world, when that comes, by flying off in the aircraft! But where would they fly to? Whatever their reasons however, they have kept the aircraft in pretty decent condition over the years, so all credit to them for that.
I do hope one can be formally preserved, indoors (the one in Cordoba is outdoors) some time soon, before it's too late.
And here is a link to all my photos of the aircraft referred to above:
Douglas DC-7Cs - a set on Flickr