but USSR never thought that bombers are a main delivery tool
Partially correct, the USSR never didn't really consider the bomber a strategic delivery method against the continental USA. They sure spent a lot of effort and resources on bomber development though as a delivery device for specific tasks (anti-CV work) or to work in continental Europe.
Consider:
Tu-95 Bear (and all the zillion variants)
Tu-22 Backfire (again, many variants)
Tu-160 Blackjack (with no conventional role)
Bison
Tu-16 Badger
Tu-22 Blinder (same designation as Backfire but earlier).
All had/have nuclear capability.
All were attempts to create a bomber of sufficient survivability and range to be used as an atomic bomber. Several inherited other roles as time passed but all were expected to carry out the nuclear role. Most lacked the range for hitting anything not in Europe as it turned out except on a 1 way trip. The Tu-22 would have been a squeak to stop in Europe though with it's high speed and long range weaponry.
The USSR did put high value on bombers as delivery weapons, maybe not against the continental US though until the Tu-160 showed up (of which there are about the same number of B2's now). These only list the major bombers that made it into service too, there were many more that did not.
-Soda
The Assassins.