Passage: The idea saw its first iteration in the Burya high-altitude jet aircraft, which reached the prototype stage. Several test flights are known, before it was cancelled by order of the Central Committee. The Burya had the goal of delivering a nuclear payload, presumably to the United States, and then returning to base. The cancellation was based on a final decision to develop ICBMs. The next iteration of the idea was Zvezda from the early 1960s, which also reached a prototype stage. Decades later, another project with the same name was used as a service module for the International Space Station. After Zvezda, there was a hiatus in reusable projects until Buran.

Question: What is the farthest stage the aircraft got?
the prototype stage