Not sure. I think it might be a lifting-body principle like the Pelican, along with some thrusters on the underside. To be honest, I have no definite idea.
A good modern exaple to look at would be the Harrier Jump Jet and F-35B. Just imagine it on a much larger scale.
Mabe they use thrust-vectoring systems like (as stated above) on the Harriers V-TOL jets and pelicans. Also Im just expanding on the entery above's answer.