A question I have following on from this discussion is this: what were the salient points that encouraged airlines to buy 707s rather than DC-8s, or DC-8s rather than 707s, when they were both fully in production?
I've only recently discovered that two stretched versions of the 707 were envisaged, with 45 and 55 foot fuselage extensions, making them 197 and 207 feet long, and a 10 foot wider wing span. Now they would have been quite a sight, the ultimate 'flying pencil'.
It's odd when I think back to when I used to see 720s coming into Heathrow, I used to think that they were quite large aeroplanes; nowadays, I think that 737-800s, which are only seven feet shorter than the 720, and the 737-900s, which are two feet longer, are quite small. I suppose this perception is because the average size of airliners has increased somewhat over the last 50 or more years.
As I said earlier, the high altitude-high speed performance of the 707 was definitely superior to the DC-8. I believe the range and payload of the 300 series was better as well. A lot of airlines were partial to Douglas, however, and the DC-8 being 6 abreast while the initial offering of the 707 was 5 abreast won some initial orders for Douglas. The other factor was that Boeing was much more successful in ramping up production, and was able to deliver with much better predictability. Donald Douglas was a great engineer, but not a great businessman, and when he completely underestimated the demand for the DC-9, he basically bankrupted the company trying to meet it and was forced to seek a merger partner. And he ended up with the worst one possible.
Stretching the 707 was much more difficult than the DC-8 because of the aft fuselage design. The 707 tapered along the fuselage axis, while the DC-8 held the top of the fuselage straight and tapered up to that line. This gave the DC-8 much more rotation clearance without having to lengthen the landing gear. But the real reason the 707 was never stretched was that Boeing was already thinking about the 747 by the time the issue of stretching the 707 came up.
As to the size of airliners, consider that there are regional jets now that are larger than the largest prop powered airliners.
What do you mean by "tapered around the fuselage axis?" The only downward taper to the 707 is the 2-3 inch taper at the fin leading edge, which is where the wider 707 fuselage tapers back into the KC-135 tail, then it goes straight aft until the tailcone.
Regarding the further-stretched 707s, I've seen models of those from the Boeing archives, they look pretty goofy.
Not sure if all of you have read "Legend and Legacy" as well as Sutter's book, both are good reads. The 367-80 wasn't quite as big of a secret as many think it was decades later, the real trick was getting USAF to short-circuit the KC-135 program to secure sales, and then turn and sell it to the airlines. There's a wonderful chapter in "Legend and Legacy," aptly named "a matter of inches," that discusses the 144" KC-135 fuselage and Douglas leapfrogging to 147. PA made a split buy, and CR Smith sat Bill Allen down and told him that they'd buy if they went to 148, so they reluctantly did, which cost about a year and gave Douglas a fighting chance to catch up. The next thing that PA and AA did was pressure Boeing into a 10-foot stretch (the original was to have the same length as the KC-135), which closed much of the passenger shortfall to the DC-8, but pissed off Qantas, who needed the lighter weight for Nadi. Boeing ended up honoring their commitment to the original-length design, with Qantas being the only customer for it.
As to why airlines bought one over the other once in full production, the edge mostly went to the 707 because it was a bit faster and had more cabin length in the 300s than the DC-8-10/20/30/40/50. Furthermore, the 707-300B/C were a bit better than the 50s in terms of payload/range, so they tended to outsell them in the 1962-65 time frame. When Douglas finally did the 62, it was a bit better on range than the 320B Advanced, but by then it was too late. The 61/63 sold well because the 707 had short gear and couldn't compete without a big rebuild, which triggered the 747. The other thing that someone alluded to was the production issues that Douglas started having in ~1966, which may have scared a few airlines into buying 707s in the late 1960s.
Lastly, because Boeing was the new kid on the block, they bent over backwards to offer customizations to people (2 TCs for AA, backward switches for TW, JT4As for Braniff, new name for a lightweight version for UA, etc.), which is why few 707s were alike. By 1963, they were somewhat more standardized, and they steadily cut down on differences throughout the generations to reduce production complexity.