Tristar and DC-10 were built to meet the same design task:
On a hot summer day, they had to take off from La Guardia notoriously short runway with full payload and fuel for trip, with safety margin for one engine out
and on a hot summer day, they had to take off from Denver, with full fuel and payload, and in case of one engine out, still cross the Rockies.
Both Lockheed and Douglas started to design twinjets towards those targets in 1966 or so, then gave up trying and chose trijets.
A300 program started 1966-1967, simultaneously with Tristar and DC-10 - and they stuck to twin.
Later on, Boeing joined the twin widebody production with 767.
Can A300 meet the La Guardia and Denver requirements? Can B767? What about A330? Or B777?