Here's The Sad Story of the Biggest and Fastest U.S. Bomber Ever. With the growing realization that Soviet SAMs posed an increasing threat to American bombers, the Pentagon started to explore low-level penetration as an alternative. Low-level penetration involved flying under the radar horizon using terrain to mask a bomber’s approach, which greatly reduces enemy response times. Moreover, the development of intercontinental ballistic missiles greatly reduced the United States’ reliance on manned bombers. Many leading military strategists of the time believed bombers were too vulnerable to survive the journey into Soviet airspace. As a result, President John F. Kennedy decided to cancel the XB-70 as a frontline bomber program on March 28, 1961.