Moussaoui told a court in Virginia on Monday that he was supposed to seize the plane with Richard Reid, who was arrested in December 2001 when he attempted to detonate a bomb in his shoe aboard an America Airlines flight.
Previous statements from Moussaoui only said the White House attack was to come later if the United States refused to release a radical Egyptian shaikh imprisoned on earlier terrorist convictions.
Moussaoui, a French citizen, told the court he knew the World Trade Centre attack was planned and that he had lied to investigators when arrested in August 2001 because he wanted it to happen.
"That's correct," Moussaoui said when the prosecution asked if that was why he misled them.
'Bit of fun'
The statement was key to the US government's case that the attacks might have been averted if Moussaoui had been more cooperative following his arrest.
He told the court he knew the attacks were coming some time after August 2001 and bought a radio so he could hear them unfold.
Specifically, he said he knew the World Trade Centre was going to be attacked, but asserted he was not part of that specific plot and did not know the details.
Asked by his lawyer why he signed his guilty plea in April as "the 20th hijacker", Moussaoui replied: "Because everybody used to refer to me as the 20th hijacker and it was a bit of fun."
Before Moussaoui took the stand, his lawyers made a last attempt to stop him from testifying, but failed.
Defence lawyer Gerald Zerkin argued that his client would not be a competent witness because he has contempt for the court, only recognises Islamic law and therefore "the affirmation he undertakes would be meaningless".
Moussaoui at first denied he was to have been a fifth hijack pilot on 11 September but under cross examination spoke of the plan that would have seen him attack the White House.
He said Reid was the only person he knew for sure would have been on that mission, but that others were discussed.
"Everybody used to refer to me as the 20th hijacker and it was a bit of fun"
He also said he talked with an al-Qaida official in 1999 about why a 1993 bombing at the World Trade Centre failed to bring the towers down.
He said: "I was asked in the same period for the first time if I want to be a suicide pilot and I declined."
Prosecutors argue that Moussaoui thwarted a prime opportunity to track down the 11 September hijackers and possibly unravel the plot when he was arrested in August 2001 on immigration violations and lied to the FBI about his al-Qaida membership and plans to hijack a plane.
To win the death penalty, prosecutors must first prove that Moussaoui's actions were directly responsible for at least one death on September 11.
If they fail, Moussaoui would get life in prison.