The Pentagon conducts the first of two training exercises called MASCAL, which simulates a Boeing 757 crashing into the building.
Charles Burlingame, an ex-Navy F4 pilot who worked in the Pentagon, participates in this exercise before retiring to take a job at American Airlines, where, less than a year later, his Boeing 757 allegedly crashes into the building.
http://www.usatoday.com/news/washington/2004-04-18-norad_x.htm
"In a third scenario, the target was the Pentagon — but that drill was not run after Defense officials said it was unrealistic, NORAD and Defense officials say."
What happened at the Pentagon to the best of my knowledge is this: At 8:20am EDT, Flight 77 (the decoy) took off from Dulles International Airport is to be involved in a drill (Global Guardian '01) demonstrating the anti-hijacking system -- the Boeing Uninterruptible Autopilot -- Canada was selling the USA. The goal of the drill ... to successfully, electronically "hijack" Flight 77 then land it safely to a predetermined location.
It involved two teams -- red team attackers and blue team defenders. The "target" of this drill is simulating a 757 crashing into the Pentagon. Flight 77 was a member of red team attackers acting as the "suicide plane." The blue team defenders were inside the Pentagon.
At 8:54am EDT, in the vicinity of Wright-Patterson Air Force Base (which happened to be participating in the Global Guardian '01 war game,) red team "hijacks" Flight 77 electronically by the Boeing Uninterruptible Autopilot through KU bandwidth via Wide Area Augmentation System and starts to head south then east.
Red team turns Flight 77's transponder off at 8:56am EDT.
As Flight 77 heads back east, while in Wright-Patterson Air Force Base airspace, a drone (a Raytheon A-3 Skywarrior in American Airlines 757 colors,) rendezvous behind Flight 77. Minutes later red team takes Flight 77 out and over Warning Area 386 (W-386) airspace and remotely detonates it.
The drone takes the place of the decoy (Flight 77.)
Blue team is oblivious to the fact that red team switched planes. Thinking it was Flight 77, the plane got within three minutes of the Pentagon. Captain Gerald DeConto, a member of blue team, recognized that a double-cross had taken place as Flight 77 (really the drone,) was supposed to turn around fly back to land at a specific location.
At around 9:37am EDT, DeConto was on the phone with Secretary of the Navy Gordon R. England attempting to get permission to launch defensive weapon(s) to defeat the A-3 as an AGM-12C missile flew through his window of the US Naval Command Center followed by the drone killing him instantly.