OK, here are some clues, so, let's see if it can be solved:
Designed to operate in locations where it's greatest competitor couldn't - only for that condition to be changed, leaving it at a disadvantage.
Originally had one fewer engine
Ostensibly lower economics due to fuel, but, high availability countered this
Famous for it's military roles, especially one, where it's probably the best in this specialist class
Almost became a bomber! Also, a double-decker version was seriously proposed.
Came in two distinct types
It might have had engines from an export version America's top fighter of the period
Noted for being quiet inside, and partially for the same reason, it became a very good aircraft for this specialist role.