Mutually Assured .... Madness!

Wiki:

The Supersonic Low Altitude Missile or SLAM (not to be confused with the U.S. Navy's current Standoff Land Attack Missile) was a cancelled U.S. Air Force project conceived around 1955. Although it never proceeded beyond the initial design and testing phase before being declared obsolete, it represented several radical innovations in tactical aircraft, some of which are now considered at the cutting edge of military technology. It was nicknamed The Flying Crowbar for its conceptual simplicity and structural strength.

The SLAM was designed to complement the doctrine of mutually assured destruction, and as a possible replacement for or augment to the Strategic Air Command system. In the event of nuclear war it was intended to fly below the cover of enemy radar at supersonic speeds, and deliver thermonuclear warheads to roughly 26 targets.

The primary innovation was the engine of the aircraft, which was developed under the aegis of a separate project code-named Pluto, after the Roman god of the underworld. It was a ramjet that used nuclear fission to superheat incoming air instead of chemical fuel. Project Pluto produced two working prototypes of this engine, the Tory-IIA and the Tory-IIC, which were successfully tested in the Nevada desert. Special ceramics had to be developed to meet the stringent weight and tremendous heat tolerances demanded of the SLAM's reactor. These were developed by the Coors Porcelain Company. The reactor itself was designed at the Lawrence Radiation Laboratory.

The use of a nuclear engine in the airframe promised to give the missile staggering and unprecedented low-altitude range, estimated to be roughly one hundred and thirteen thousand miles (almost 182000 km or over four and a half times the equatorial circumference of the earth). The engine also acted as a secondary weapon for the missile: the stream of fallout left in its wake would poison enemy territory, and when its fuel was spent it would severely contaminate its strategically-selected crash site. In addition, the sonic waves given off by its passage would damage ground installations.

Another revolutionary aspect of the SLAM was its reliance on automation. It would have the mission of a long-range bomber, but would be completely unmanned: accepting radioed commands up to its failsafe point, whereafter it would rely on a Terrain Contour Matching (TERCOM) radar system to navigate to preprogrammed targets.

Although a prototype of the airframe was never constructed, the SLAM was to be a wingless, fin-guided aircraft. Apart from the ventral air intake it was very much in keeping with traditional missile design. Its estimated airspeed at thirty thousand feet was Mach 4.2.

The SLAM program was scrapped on July 1, 1964. By this time serious questions about its safety had been raised, such as how to test a device that would emit copious amounts of radioactive exhaust from its unshielded reactor core in flight, as well as its efficacy and cost. ICBMs promised swifter delivery to targets, and because of their speed (the Thor traveled at roughly Mach 12) and trajectory were considered virtually unstoppable. The SLAM was also being outpaced by advances in defensive ground radar, which threatened to render its stratagem of low-altitude evasion ineffective.