WTC was intended to be a delayed one-two punch, a common terrorist technique to ensnare the rescue workers. No, their failure was down in DC. I can't believe the Pentagon was more of a symbolic prize than the Capitol would have been. My guess is that the 3rd, 4th, and 5th targets were in DC: Pentagon, Capitol, & White House. WH was too hard to pick up coming in from the west, perhaps the 1st circle on the Capitol was too difficult(rumor was flight school said one of these guys was a crummy pilot), so they went for the Pent., with the expectation of follow up attacks. The only thing I don't get was why they did this attack when Bush was out of town. With all the inside info they had, surely they knew he wasn't at the WH. But the idea that one of these hijacked planes would take out AF1 makes absolutely no sense.
So all I can guess is that they may have wanted him out of the WH, so as not to be in the bunker. Of course if the WH was the 1st target, that point would be mute. But my guess is that there was a ground plan for Bush, perhaps a SAM(but high risk of failure) or suitcase nuke somewhere near his return to DC. but the delayed return allowed for a tightening of security that closed that window. But this theory seems unlikely, too.