The X-Jet is what one of two aircraft the X-Men use to travel long distances from place to place in secret. These aircraft are stored inside the mansion and usually exit through a concealed hangar door in the cliffs at the edge of the Institute's grounds.
The first and largest aircraft is the SR-77 Blackbird, a large, powerful twin-engine long-range stealth jet. It is flown principally by Cyclops or Wolverine, sometimes by Nightcrawler.
The second is a smaller hybrid jet/helicopter called the X-Velocity, which is first used by the remaining X-Men and Brotherhood members to reach Asteroid M. Unlike the Blackbird, the Velocity is constructed without any metal whatsoever, making it immune to Magneto's powers. The first Velocity is shot down during the events at Asteroid M. It is later reconstructed, and is later used by the X-Men while on the run following the destruction of the Institute.