Explanation part 2 since SP has a character limit for comments: this wasn’t just another basic project, Boeing was playing around with one of the most radical asymmetrical designs ever, no, it’s not just a simple asymmetric seating format of 2-3 like some planes have, they were going to make the entire wings asymmetric, one was going to have a steeper dihedral & the other was going to have larger flaps; this insane asymmetrical design was to counter the drag that the 3rd engine would produce, however in 1985, ETOPS extended the limit to 2 hours, in 1990 when the Boeing 777-100 was almost about to start it’s production, ETOPS changed it to 3 hours; twin engine planes could now essentially fly long-haul routes. You see, 4 years prior in 1986, McDonnell Douglas knew the time limit was gonna increase so they came up with the MD-11, a more fuel efficient trijet, it was introduced to service in December 1990 but in the end, only 199 were ever built with production lasting from 1988 to when Boeing acquired McDonnell Douglas, however, they didn’t stop the MD-11’s productions, the production for the MD-11 kept going until October of 2000 which was an embarrassment compared to the 385 DC-10’s built with production lasting from 1970 to 1990 as well as the 250 L-1011’s built with production lasting from 1969 to 1985, because of this current situation, Boeing was forced to abandon the project, however, they launched it again starting production of the Boeing 777 in 1994, this time with only 2 engines, eventually in 2007, the ETOPS rule was written off completely, today, we see the Boeing 777 as a twin engine plane powered by 2 massive Rolls Royce Trent 800’s or General Electric GE90’s with the upcoming Boeing 777X powered by General Electric GE9X’s, imagine how different everything would’ve been had the concept came to fruition, eventually, all trijets were replaced with twin-engine planes in the early 1990’s. In the end, the era where trijets dominated long-haul travel lasted only 2 decades, but they’re most important legacy is in what they taught us, the balance between redundancy & efficiently isn’t static. Next time you see a Boeing 777, remember the ghost in it’s wake; a trijet that almost was but helped difine what is.

Explanation: in 1978, Boeing proposed the Boeing 777 but in a different form from how we see today’s Boeing 777’s, it was a trijet, 2 engines mounted under the wings, 1 mounted as an s-duct through the tail, it was meant to compete with other trijets at the time which were dominating long-haul flights from the early 1970’s to early 1990’s, this was because of ETOPS which was an FAA rule that prohibited any twin-engine planes from flying further than 1 hour from an airport throughout it’s entire route in case of an emergency; this meant that planes with only 2 engines couldn’t cross oceans, because of this, only planes with 4 engines couldn’t cross oceans fly long-haul routes but 4 engines made them very thirsty for fuel which costed a lot of money, because of this, McDonnell Douglas & Lockheed came up with an idea: what if they could make a more fuel efficient plane that can carry the same amount of people as 4 engine planes? That’s when they both came up with the idea for a plane with 3 engines, they both got to work. In August 1971, McDonnell Douglas introduced the McDonnell Douglas DC-10, a wide-body plane with a seating format of 2-5-2, it had 2 engines under the wings like usual & 1 engine placed through the vertical stabiliser, it instantly became a hit. In late April 1972, Lockheed rolled out the Lockheed L-1011 TriStar, just like the DC-10, it had 2 engines under the wings & a seating format of 2-5-2, however though, it’s 3rd engine was mounted inside of the tail through what was called an “s-duct”, it also instantly became a hit & together, the 2 completely reshaped long-haul air travel. Eventually Boeing announced a new trijet called the Boeing 777, specifically the Boeing 777-100; a plane with exactly the design of the DC-10 & L-1011, as there were only 2 planes at the time that were efficiently dominating long-haul flights without consuming as much fuel as 4 engine planes, airlines were limited to only 2 choices, they sincerely needed a plane with a max capacity of exactly 250 - 300 people & a range of 6500km - 8000km or 4000mi - 5000mi, that’s when Boeing came up with the Boeing 777-100.