The YX, or YS-33 was planned to be Japan's first commercial passenger Jet first envisaged in 1967. Although plans had been made by NAMCO for a jet powered variant of its YS-11 turboprop regional airliner, these were only as a stop gap until production could begin of the medium range YS-33. It was realised that Japan did not have the resources for development of such a type of aircraft, so it was decided in 1970 to approach European manufacturers to enter in a joint production and development. Although the design was initially planned for only 120-150 pax, this had increased to 200 by this time. Fokker and BAC were approached, but with the development of the Airbus A300 ongoing on the continent, Japan turned to US companies. Power was also initially three RR Trents (The original engine with that name, not the one that currently flies on both Airbus and Boeing designs), but this was changed to two larger engines under the wings. Boeing was initially the front runner in the bid for a US partner, but other bids came from both McDonnell Douglas and Lockheed. By late 1972, it was realised that Japanese Government funding would not be forthcoming for such a risky venture and so the the YX was abandoned. As can be seen, the Lockheed submission would have drawn heavily from the L1011 TriStar and the Boeing and McDonnell Douglas designs would also have, no doubt, drawn inspiration from other existing models built by those respective companies. Lockheed themselves did also look at a twin-engined variant of the L1011 the -600 model.
