B52's actually have pre-wrinkled skins since the whole device flexes. They even have small wheels on the wing tips. I heard (have not verified on the web) that the wings have 12 feet of "flap" at the tips.
Now, metal fatigue is an issue and they do check for it during inspections. Generally it becomes visible before it becomes an issue with the exterior parts. It actually is far more likely for fatigue to affect the engines since heat is a much larger component.
Fuselage and wings are more like the leaf springs. They use alloys (and now-a-days composites) that cant take a lot of cycles before the material fails.
And if God had wanted man to fly, he would have given him more money!
2011 PrimeTime Tracer 2600rls
2001 GMC Yukon XL 2500, 6.0L 4.10:1 axle ratio
Nights camping 2014 - 17
2013 - 15