Two planes that come to mind that were never never produced were the Boeing C-14 and the Northrop F-20. The Boeing had top-mounted engines which, so I was told, provided better lift and less chance of FOD on unimproved air strips. Program cancelled. The F-20 was to be built primarily for overseas customers but when the Air Force showed no interest, foreign governments shied away from it. A good, proven design but no sales.
I don’t know why anyone would buy an F-5 retread (F-20) when they could be mainstream and go with the F-16 or F-15.
F-20 was also a single engine plane. At the time no Middle East pilots would fly a single engine aircraft, they didn’t trust them. They’ve gotten past that barrier now.