By using the immense heat from an on-board nuclear reactor, the Air Force postulated the plane could take off (at great speed) and then remain aloft and at speed almost indefinitely. They further postulated that the enemy wouldn't dare shoot them down lest they basically bomb themselves with the aircraft's power system. Ultimately a few attempts were made to make flying reactors possible, but crew shielding and protection on the ground proved more difficult and costly (1 Billion Dollars in the 1950's!) than the plans merits, and the project was shut down.
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