The proposed civilian Cargolifter 160 has a capacity of 160 metric tons, which could lift on the close order of three Abrams tanks (a C-5 can carry one). It would also have the capacity of dropping them somewhere where there is no airport at all, and has a range of about 10,000 km, about twice that of a C-5.

I'm not sure this is sufficient advantage to make up for its putting along at about 60 mph, but it is something to think about.

The Cargolifter company is not doing that well, and I don't know if anything like this will ever be built in the civilian world. If something like this has been built secretly by the military, I have no idea how the hell they would hide something that big.

More plausible would be the smaller ships discussed by Dan, designed for stealth reconnaissance, not cargo.