A celebration of traditional family life and values, the five-day gathering of prayer, teaching, and jubilee will promote Roman Catholic values, Chaput said, but will have an "interfaith" and "ecumenical" feel.
Corbett said the event could bring "hundreds of millions of dollars" to the region.
Nutter predicted that it would be "an incomparable moment for the city of Philadelphia" and the largest event in its history. He voiced confidence that the city could accommodate the crowds even if they reached two million.
"We're the big-event city in the United States," Nutter said. "The world's eyes will be on us, and we will shine brightly."
Asked if he and Corbett might lean on President Obama to urge a Philadelphia visit when he meets with Francis in Rome on March 27 - the day they depart - Nutter joked that "only in Philadelphia would people talk about trying to 'seal deals' with the pope."
Scheduled for Sept. 22 to 27, 2015, the World Meeting will include lectures and programs at dozens of venues across the Philadelphia area, with separate programming pitched to children and adults.
Chaput said he that expected that the largest stream of visitors would come from North and South America, but that with many coming also from Europe and Africa, it would be a multilingual affair.
Million-plus crowds are not predicted unless Francis pays a visit to the World Meeting, but it has been a tradition since the gatherings began in 1994 for popes to say Mass at the close. Pope Benedict XVI did not attend the 2009 World Meeting of Families in Mexico City, however.
The 2012 World Meeting in Milan, Italy, drew about 350,000 people from 153 countries, and about a million people streamed to the city for a Mass celebrated by Benedict.
It was at that meeting that Benedict, joined by Chaput, announced that he had chosen Philadelphia for the next World Meeting.
On his return from Milan, Chaput said he hoped to limit the meeting's crowd size because of the cost and logistical demands the event might impose on his financially strapped archdiocese.
On Friday, however, he voiced confidence that the local church could manage an event of any size.
"I was very nervous about all the problems" last year, Chaput said. But since then "the response has been extraordinary" from the business and philanthropic communities, which he said had already pledged $5 million.
Chaput said he could not predict the cost of the five-day event, since it is still in planning, but he conceded that a papal visit would add significant costs, including for security.
The Vatican "does not have the resources" to contribute financially, he said, and fund-raising will ultimately be his responsibility. "We will need considerably more" than $5 million, he said, and later offered an estimate of $10 million to $15 million.
During the news conference, Chaput announced that Robert J. Ciaruffoli, chairman of the accounting firm ParenteBear L.L.C., had been named president of World Meeting of Families L.L.C., a tax-exempt non-profit created for fund-raising, planning, and logistics.
Serving on its board will be David L. Cohen, executive vice president of Comcast Corp.; Daniel J. Hilferty, president of Independence Blue Cross; James J. Maguire Sr., co-principal of the Maguire Foundation; Joseph Neubauer, chairman of Aramark; and Brian L. Roberts, chairman of Comcast.
The Philadelphia gathering will be the first held in the United States.
Noting that this will be the first World Meeting since Francis' election to the papacy a year ago, Chaput said it might have a different tone than the World Meetings under Benedict and Pope John Paul II. The Vatican's Pontifical Council on the Family will officially announce its theme at a news conference March 25.
Regardless of its theme, the event will be "very positive," Chaput said, and will seek to help families "be themselves, and grow in grace and holiness."
"All of us belong to a family," he said. "It's a Catholic gathering led by the pope, but he's loved by people around the world. I think people from all backgrounds will be happy and welcome."