The XB-70 that was lost in the mid-air was the second one built - and had incorporated lessons learned from the first
https://www.nasa.gov/centers/armstro...-084-DFRC.html
….Although intended to cruise at Mach 3, the first XB-70 was found to have poor directional stability above Mach 2.5, and only made a single flight above Mach 3. Despite the problems, the early flights provided data on a number of issues facing SST designers. These included aircraft noise, operational problems, control system design, comparison of wind tunnel predictions with actual flight data, and high-altitude, clear-air turbulence.
NASA Ames Research Center, Moffett Field, CA, wind-tunnel studies led engineers at North American Aviation in Downey, CA, to build the second XB-70A (62-207) with an added 5 degrees of dihedral on the wings. This aircraft made its first flight on July 17, 1965. The changes resulted in much better handling, and the second XB-70 achieved Mach 3 for the first time on Jan. 3, 1966. The aircraft made a total of nine Mach 3 flights by June….