The US owns 4 aircraft referred to as "Boeing E-4B". The Boeing E-4B serves as a mobile command center, and is known as a "Nightwatch aircraft". These aircraft are based in Nebraska, but at least one E-4B NAOC is always on 24-hour alert, 7-days a week, with a global watch team at one of many selected bases throughout the world.

On September 11th, 2001, Air traffic control recordings and radar data indicate the E-4B call sign VENUS77 became airborne just before 9:44 am, circled north of the White House during its climb, and then tracked to the south of Washington, D.C., where it held in a holding pattern. The call sign "NIGHT76" from today likely refers to the plane whose tail number ends in 76(the others are 77, 87, and 125).

The aircraft primarily used for POTUS transport is the Boeing VC-25A. They don't take the E-4B out for joy rides. It's a huge deal.

All sources for this info are easily accessible public websites.