They only built two prototypes. The second one collided with it's chase plane and crashed.
The program was scrapped when the Russians shot down Francis Gary Powers in his U-2 in 1960.
The idea of high altitude high speed penetration was gone forever. From that day on B-52 pilots were trained for low level tree top penetration in order to deliver our nuclear weapons.
High altitude high speed penetration gone forever? Are you unfamiliar with the SR71? We were just getting started!
The advancement of ICBM technology doomed the nuclear bomber as the primary delivery means for nukes. Not the loss of a U2. We don't give up that easily. You're right about shifting focus to low level penetration for bombers though. The B1 was designed specifically for nap-of-the-earth penetration.