America's ultra-rich are queuing to join in a grand gesture of generosity. Forty US billionaires have signed up to pledge at least half of their fortunes to charity under a philanthropic campaign kicked off by Warren Buffett and Bill Gates.

In an unprecedented mass commitment, top figures including New York's mayor Michael Bloomberg, the hotel heir Barron Hilton, CNN media mogul Ted Turner, and the Star Wars director George Lucas have lent their names to the "giving pledge", an initiative founded six weeks ago to encourage America's richest families to commit money to society's most pressing problems.

The pledge is not a legally binding contract but is described as a moral commitment. Inspired by the Bill and Melinda Gates Foundation which pumps billions into fighting disease in developing countries, it does not prescribe any particular charitable causes but is a statement of principle.

Buffett, the 79-year-old Nebraska stockpicker nicknamed the Sage of Omaha, who has a $47bn (£29.6bn) fortune, said the aim was to generate peer pressure encouraging billionaires to take an aggressive approach to philanthropy. "We're hoping America, which is already the most generous society on earth, becomes more generous over time," he said.

To imbue team spirit, Buffett plans regular get-togethers for his fellow billionaires, kicking off with a series of dinners for 15 to 20 people at locations around the US this autumn.

On the face of it, the sums involved are enormous. Among those committing to give away money are the Oracle business software tycoon Larry Ellison, whose fortune is estimated by Forbes magazine at $28bn, the banker David Rockefeller ($2.2bn), oilman Boone Pickens ($1.1bn) and private equity tycoon Pete Peterson ($2bn).

Also on the list are the media entrepreneur Barry Diller and fashion designer Diane von Furstenberg, his wife. A former Citigroup banking boss, Sandy Weill, has signed up, as have fellow Wall Street names including David Rubenstein, co-founder of the Carlyle private equity group.

Experts were asking how much being pledged was new money, as opposed to wealth already committed to charitable foundations.

"I think it's remarkable that so many people have agreed to go public with their commitments," said Stacy Palmer, the editor of the Chronicle of Philanthropy in Washington DC, which covers the nonprofit world. "But I'll be more convinced that this is truly transforming philanthropy when I see names on the list who aren't the usual suspects."

Buffett and Gates have been banging the drum for the initiative by contacting billionaires, one by one, asking them to lend their names. So far, roughly half of the 70 to 80 individuals approached have agreed to pledge money, with some promising considerably more than the minimum 50% of their wealth.

Bloomberg, who amassed $18bn through his eponymous financial information empire, said wealth reached a point where billionaires have so much that they simply cannot spend it. It did not make sense to leave all his money to his children so that they could become members of the "lucky sperm club".

"If you really care about your family, it's best to do something to make the world a better place for your children and grandchildren, rather than just giving them money," said Bloomberg, whose charitable interests include anti-smoking campaigns and road safety.

A clutch of America's top businesspeople have joined the pledge including construction tycoon Eli Broad, former Cisco electronics chairman John Morgridge and hedge fund boss Julian Robertson.

There are notable absentees.

Four of America's 10 wealthiest people are members of the Walton dynasty which founded the world's biggest retailer, Walmart, but none of the discount shopping heirs has signed up to the pledge. Neither have the founders of Google, Sergei Brin and Larry Page, who are worth a combined $30bn through internet search and advertising.

Some are sceptical of the way Gates and Buffett are creating a highly public philanthropic elite.

Pablo Eisenberg, a senior fellow at the Georgetown Public Policy Institute, at Georgetown University, Washington DC, said ultra-wealthy donors tend to give money to higher education, arts and established healthcare causes, with relatively little going to poverty reduction, disability causes or to disadvantaged ethnic minority communities. Billionaires generally gave away funds through tax advantageous foundations.

"These mega-foundations, which are effectively family enterprises with no accountability, are going to dictate public policy priorities for this country," said Eisenberg. "I'm not sure that tax receipts haven't done a better job, over time, of meeting the needs of our neediest people, than philanthropists."

Publicly declared philanthropy is much greater in the US than Britain, where wealthy individuals tend to shy away from the public gaze.

Bea Devlin, head of international policy at the Charities Aid Foundation in Kent, said the top 100 philanthropists in Britain last year gave away an estimated £3.5bn. She said she would welcome more "open champions" among Britain's super-rich to encourage philanthropy but she added: "Giving just isn't in that boastful space in the UK at this point."