The United States has initiated many fighter jet programs only to abandon them later on. Sometimes a jet design was reworked and entered service as a very different type of fighter jet. Perhaps the most famous combat aircraft the United States flew but then abandoned was the North American Aviation XB-70 Valkyrie (however, that was a bomber that was abandoned in the 1960s and so is outside the scope of this article). Here are seven fighter jet designs that were abandoned (some were proposed improved variants that the DoD declined).