The Convair B-36 was SAC's primary nuclear bomber from 1946 to 1959. Based in Alaska, they had insufficient range to fly the return leg of a bombing mission over the Soviet Union. In addition Operation Castle's results indicated that a typical wartime delivery profile would result in significant damage to the aircraft. Either way, it was a one-way trip.

With the introduction of the B-52, this changed. It was faster, longer ranged, and capable of in-flight refueling. SAC had up to 12 of them in the air at any given time. Assuming they weren't blown out of the sky by SAMs or Interceptors; they could make it back to friendly soil.

It really depends on when and what country you're talking about. Can you clarify?

Does anyone have an answer for Soviet bombers?

Or maybe even French, British or Chinese bombers pre-missiles?

The very excellent 15 Minutes: General Curtis Lemay and the Countdown to Nuclear Annihilation (which incidentally I picked up based of the recommendation of a flairred user here, /u/restricteddata) address this at one point.

If a nuclear exchange was preceded by a period of heightened tensions the United States Air Force's Strategic Air Command (SAC) would have increased the number of aircraft it had on air borne alert, and also farm out the remaining bombers across the continent to a series of secondary and tertiary fields. Theoretically some of this alternate air fields would have survived a initial exchange, especially given that SAC expected a nuclear war not to be fought with one massive bang and its done exchange but to fight nuclear was more akin to the tides- every few hours horrible, horrible things would happen. The theory is that a returning bomber would be able to find one of these tertiary fields that would allow them to refuel and rearm and do it again. That is all incredibly theoretical, as realistically SAC knew the vast majority of it's bombers would have been shot down or landed elsewhere. The survival rate on the B52 crews would be low enough to make an attack on the USSR a suicide mission.

In the case of Strategic Air Command, the crews of the B-52 were always trained to be prepared to ditch their planes after reaching their targets. The plan that SAC had drawn out was that a B-52 would launch from its forward located base, receive usually one air refuel then proceed to its target. Ideally, the plan would be that once they delivered their pay load, they would return home and most likely receive another air refuel on the way.

Realistically, SAC was prepared that the B-52 would at least reach its target and it was unlikely that there would be a refueler to meet up with them. They were directed to attempt to ditch in friendly or neutral territory. The idea of ditching wasn't lost on the Boeing either, the first few generations of B-52 were utilitarian, no comfort, and had a as one B-52 Nav that I knew put it "a disposable feel to it."

If you are really interested in this topic, I recommend reading Command & Control by Erich Schlosser. A book so accurate that the Pentagon Joint Staff had an investigation.

For the British I recall a documentary in which an ex-RAF pilot said and I quote "we expected to pass the target and continue to fly deep into the East, may be Siberia where we hope to settle with a large Siberian woman" British humor all around it clearly implies the fact that they knew it was one way ticket.

This is discussed in the book Command and Control by Eric Schlosser, which I'm currently reading. For exactly this reason, H-bombs were equipped with parachutes. If the bombs fell directly to the ground and detonated the plane would be destroyed; slowing their descent with parachutes gave the bombers time to get away.