If a B-52 bomber were flying at its normal altitude and not producing any noise, it would be impossible to hear it regardless of the distance. Hearing requires the presence of sound waves, which are mechanical vibrations that propagate through a medium, such as air, water, or solids.
In the absence of sound waves, there would be no acoustic energy reaching our ears to be perceived as sound. Even if the B-52 were flying relatively close to the ground, without the sound of its engines or any other noise-producing factors, the absence of sound waves would render it completely silent.
Therefore, in the scenario you described, where the B-52 is not making any noise, you wouldn't be able to hear it regardless of the distance.