The 7J7 was going to use tail mounted unducted fans - basically pushing for max fuel efficiency. They went as far as to do a demonstrator aircraft (I'm thinking it was a DC-9 but don't hold me to that), replacing one engine with the unducted fan prototype. I don't know how fuel efficient it was, but the counter-rotating props were so noisy it quickly became a show stopper.
The Boeing SST was going to be called the 2707 before it was cancelled, so I suspect the next new aircraft after the 797 will be the 1707.