The survivability of each bomber on a mission, at least until reaching the target, would also be important.
In 1952 the recon versions of three types of nuclear-capable bombers, the NA B-45C, the EE Canberra B.2 and the Boeing B-47B â€“ the only one listed in this thread â€“ performed deep penetration missions of the Soviet Union.
The RB-45C's flew daytime recon-missions, with USAF crews, over the Soviet Far-East and night-recon missions, with RAF crews, over Moscow and the Ukraine.
A Canberra B.2, equipped with a US-made, heated, 8-camera pod in the bomb bay flew another daytime recon mission over the Soviet missile facility at Kapustin Yar, under Operation Ju-Jutsu. The aircraft was damaged by a MiG-15, but managed to reach safety.
A B-47B, also with a camera-pod fitted in the bomb-bay flew another daytime recon mission on the Soviet Far East, flying out of Alaska. MiG-15â€™s attempted to intercept it, but having been scrambled too late, never managed to reach the bomber, which made good its escape.
I may be wrong, but it seems to me these missions clearly demonstrated the survivability factor â€“ at least in the early fifties â€“ of these three types and, to my knowledge, no other type ever got this close to a â€˜realâ€™ Cold-War operational test.