Originally Posted by
ROCAT
You could on DC-9s, B727s and I think on the Tridents as well.
I was actually thinking of the Ilyushin 86 widebody, where the entrance was below the main deck. There was a baggage counter. Passengers entered with their bags, handed them over, got a check, then went upstairs to the seating. Did away with much of the airport infrastructure (high steps/jetways; baggage handling).