“Built especially for intercepting the SR71”
From ‘MIG Pilot’ Victor Belenko’s biography, he mentions the B-70, which he was briefed on, and was told there was a ‘special project’ that would take care of that. That became the MIG 25.
As I recall from Victor Belenko’s biography, when he landed at Hikido Japan he found out the ejection seat mechanism was not there. It was filled with avionics equipment.