The sound barrier is 1240km/h, if the B52's top speed is 638mph, that's only 1020km/h. That pilot would need an extra 18% on the throttle for that to happen.
I'm not an aviator, but my wave mathematics is pretty good. In order for the mach cone to form around the plane, you actually have to be traveling at 340m/sec. If your anything below that, then a half-assed cone forms infront of the plane.
Here's a good link to play around with
For those wondering exactly what's happening. Lemme explain as best as I can. As the plane cuts through the air, it produces shock waves, some of these are audible and some not. Now sound travels in a spherical shape from it's source. So when your flying, the speed of the waves extending in front of you is less than the speed of sound (340m/sec). As your forward velocity approaches that barrier, the waves begin to superimpose, or stack up, in a standing waveform. This is what produces the mach cone. Now what you actually see is not the sound, but the enormus pressure wave compresses the surrounding air enough to condense the water vapor into liquid. Play around with that link and it should all become clear.