Pratt & Whitney’s first-generation turbofan was the JT3D (a derivative of the company’s first-generation turbojet, the JT3C), which entered service in March 1961, and, due to its greater power and lesser noise and fuel consumption, quickly replaced the JT3C (and also P&W’s second-generation turbojet, the JT4A) on the aircraft that had previously relied on the JT3C and JT4A (two civilian [the 707 and DC-8]1 and two military [the B-52 and KC-135]).2
However, the JT3D, although a vast improvement over the loud, inefficient turbojets it replaced, was still very much a first-generation design, with problems not seen on newer turbofans, and still quite the screamer and kerosene-guzzler even by the standards of just two years eleven months later, when Pratt & Whitney came out with their second-generation turbofan, the JT8D (entered service February 1964 with the 727). The JT8D was a vast step ahead of the JT3D, and is still practically the state of the art for civilian low-bypass turbofans;3 not coincidentally, it is also the best-selling low-bypass turbofan ever, and the second-best-selling jet engine of any kind in history.4
Yet, even as Boeing and (McDonnell) Douglas readily adopted the newer, better-designed, quieter, more efficient JT8D for their second-generation jetliners (the 727, DC-9-10-through-50,5 and 737-100/-200), they continued to use the old JT3D on their first-generation jetliners for as long as those aircraft remained in production; JT3D-powered DC-8s continued to spawn in Long Beach until 1972, and the guys and gals from Washington State kept on putting JT3Ds on new 707s all the way up until 1979. Admittedly, the early models of the JT8D had somewhat less thrust than the JT3D, but they could easily have been upbeefed to JT3D-equivalent power levels - as was, indeed, done in the 1970s to create the JT8D-200!
Why did Boeing and (McDonnell) Douglas continue to use an old, outdated engine for the 707 and DC-8, even after the JT8D came along?
1: Technically, not all pre-JT3D 707s and DC-8s used a P&W turbojet (the 707-420 and DC-8-40 both used the Rolls-Royce Conway, the first turbofan ever to fly), but the vast majority did (as U.S. airlines were reluctant to buy airliners with foreign-made engines, meaning that most orders of Conway-equipped 707s and DC-8s were for the smaller European and Canadian markets).
2: The turbojet variants of these aircraft were the 707-120, DC-8-10, B-52A-G, and KC-135A (JT3C) and the 707-220/-320 and DC-8-20/-30 (JT4A); the JT3D powered the later 707-120B/-320B/-320C, DC-8-50/-60, B-52H, and KC-135E.
3: Not that that’s saying much, given that the market for civilian low-bypass turbofans has (with one notable exception) been essentially extinct since the first narrowbody-sized high-bypass turbofans broke into the market in the early 1980s.
4: ...behind only the engine that finally evicted the JT8D from its niche, the General Electric/SNECMA CFM56 high-bypass turbofan.
5: Along with the DC-9-80, when it came along (which happened, ironically, shortly before the JT8D was rendered obsolete by GE’s French connection).