This situation has been poorly reported, and I don't think many people without a security clearance understand what's been happening in the past few years. The threat is a Russian decapitation strike. Non-alert bombers are trivially easy to destroy. The ICBM fleet now depends on 40 launch control centers, or one of a few special aircraft in order to be able to launch. These aircraft, prior to 1991, were on continuous airborne alert. There are now three of them on ground alert, with (my guess) 15-30 minute response time.
These aircraft are also the primary communications mechanism with the missile subs. The subs are also capable of raising a communications buoy and communicating via satellite, but this is not normal operating procedure, and there are only about five satellites that they can use.
The Russians have a number of new weapons systems, some public, some not. They appear to have a maneuvering reentry vehicle that could allow depressed trajectories, which means sub-10 minute warning times. This is not enough time for the President to be located, make a decision, transmit it to the silos, and have the missiles away. There are also several new Russian anti-satellite weapons, including one that appears to be in geosynchronous orbit, which could knock out the communications satellites to talk to the subs. With a communications blackout, there would potentially be time to destroy the subs before they could launch.
Having bombers on alert is not a real solution to this problem, but it's better than nothing. Potentially, the bombers could be ordered to launch at the first warning, and might make it away from the airfield in time. They could start flying to targets immediately, but wouldn't have to strike if it were a false warning. The better solution would be to put the special planes (the E-6Bs) back on continuous airborne alert, but according to what the generals said at some congressional hearings back in March, the problem is that the E-6Bs are too old, and there aren't enough of them to be able to maintain airborne alert. Unfortunately we don't even have a budgeted program to design a replacement, and the airframe they're based on is obsolete.