So even given the best situation, NEADS knowing the exact position of Flight 77 when launching the Langley Fighters, they likely still would have only gotten there in time to see Flight 77 hit the Pentagon.
The Langley fighters were airborne at 9:30 so if they knew the exact position of Flight 77 they had less than 7mins to close within AIM-120 AMRAAM range, which was probably between 45-65 miles. I'm assuming they were armed with this missile because the AIM-54 Phoenix was only used by the US Navy for long range fleet defense, obviously primarily to protect carrier battle groups.
Even given this best situation scenario I still don't see how they could have shot down Flight 77 before it went low and lined up to hit the Pentagon.
Maybe Reheat, Cheap Shot, or Gumboot could confirm this?
That's pretty much it. Assuming a host of factors that make an intercept more likely, based on the warnings NORAD was given an intercept of the first three flights was still impossible.
Those factors might include:
1. Confirmed and precise location of hijacked flights
2. Awareness of the nature of the threat the flights posed
3. Authorisation to immediately engage non-responding airliners without prior ID and interception
Of course if you're talking hypotheticals, you could argue "What if NORAD were told about the hijackings immediately, knew what the hijackers intentions were, and had permission to fire?"
To that I usually respond "What if the US Customs officials that met the 19 hijackers when they arrived in the US knew they were members of Al Qaeda and planning to carry out a terrorist attack?"
What ifs are pointless. There's what happened, and that's all.
-Gumboot
ETA. Spin, you're quite correct regarding armament. The AIM-54 Phoenix was specifically designed to be carried by the F-14 Tomcat in conjunction with the AWG-9 radar. No other aircraft is capable of carrying the Phoenix, and even if they had it mounted, without the AWG-9 radar they could not use the weapon.