It capped a six-week process of elimination that always pointed in Biden's direction even if the Delaware lawmaker couldn't believe he would actually be the pick.
The Democratic ticket, revealed Saturday in Springfield, Ill., began to form when Obama approached Biden in July and asked him if he'd consider being a contender for vice president, according to two Biden associates.
Biden said he would and submitted the requisite paperwork to the vetting committee, but was skeptical that it would ever happen. A longtime friend who requested anonymity to speak freely said Biden wouldn't allow himself to believe he was in serious consideration because his hopes and expectations had been beaten down so much by such a miserable showing in the Democratic presidential race.
In the end, it came down to Biden, Indiana Sen. Evan Bayh and Virginia Gov. Tim Kaine, said several people involved in the deliberations. Kansas Gov. Kathleen Sebelius also was a strong contender, but dropped out of contention as Obama focused on the other three.
Hillary Rodham Clinton, who ran so closely to Obama in the primary, was never seriously considered, said two officials involved with the search. She asked not to be vetted unless she was going to be picked, the two officials said, speaking on a condition of anonymity to describe the private discussions.
In the final days before the announcement Obama's staff only prepared for an announcement of Biden, Bayh or Kaine - and Obama had made his decision by Thursday and called them all that day to tell them their fate.
Obama campaign strategist David Axelrod described the search as "a long process but it always pointed in Biden's direction."
Illinois Sen. Dick Durbin said he talked with Obama about potential candidates three weeks ago on a campaign bus swing through Florida. They made pro-and-con sheets and speculated about how the candidates would respond to various scenarios.
"Many others were discussed but my impression was that those three a few weeks ago were really the centerpiece - Kaine, Bayh and Biden," Durbin said. He added that he didn't learn of the pick until Saturday morning, although he suspected which direction Obama was going.
Biden clearly was the leading contender by Wednesday night, when he told reporters staked outside his house, "I'm not the guy." Biden's associates said he knew at that point that he would probably be the pick, but said that because he hadn't gotten the word yet.
From all accounts, Biden told just a handful of close friends and family he was being considered. They included his wife, Jill; three children, Beau, Hunter and Ashley; sister and political adviser Valerie Biden Owens; and longtime Senate chief of staff Ted Kaufman.
Kaufman said Biden had genuine respect for Obama and that the two had hit it off immediately after Obama came to the Senate. In Indiana, finalist Bayh finally could relax Saturday once the rest of the world knows what he had known for two days but couldn't repeat - even as a report of Obama-Bayh bumper stickers being printed fueled speculation he was the pick. Bayh said he had to keep quiet as friends called to congratulate him Friday.
He said he told his wife, "We made the final three in a competition where they don't give out silver and bronze medals." And he said he's asked his staff to track down some of those bumper stickers for his scrapbook.
Associated Press writers Christopher Wills and Beth Fouhy in Springfield, Ill., and Mike Smith in Indianapolis contributed to this report.