The F-106X (Model 8-28/8-29) was a 1956 design study for a Delta Dart follow-on. This study envisaged an interceptor with a canard layout that was powered by a JT4B-22 turbojet fed by rectangular air intakes. It was envisaged as an alternative to the Lockheed YF-12, and was to have had a fire control system with "look-down, shoot-down" capability fed by a 40-inch radar dish. The F-106X project was extremely advanced for its time--some references state that a Mach 5 performance was envisaged. However, this is probably vastly over-optimistic. Mach 3 is probably more like it--although still advanced for the time.
The project was later redesignated F-106C/D, with C being the single-seat version, the D being the two-seat version. At one time the Air Force had considered acquiring 350 of these advanced interceptors, but the F-106C/D project was cancelled on September 23, 1958.
While contemplating the cancellation of the Avro Arrow project, the Canadian government briefly considered the possibility of purchasing the F-106C/D. However, the USAF cancelled the F-106C/D before this idea could be pursued any further. After the eventual cancellation of the Arrow program in February of 1959, the Canadian government decided to acquire the CF-101 Voodoo instead.
Following the cancellation of the Model 8-28/29 project, two production F-106As (57-239 and 57-240) were modified to test the new radar housing with a five-foot nose extension. They were redesignated F-106C. Only 57-239 actually flew, and made ten flights with this new nose in 1959. The plane was later destroyed in fatigue tests. 57-240 eventually reverted to standard F-106A configuration.