At a press conference, Michael Palin, Eric Idle, John Cleese, Terry Gilliam and Terry Jones said they wanted to see if they "were still funny".
Idle - who will direct - said the audience should expect "comedy, pathos, music and a tiny piece of ancient sex".
The stage show will be their first new project for three decades.
It is more than 30 years since the Pythons last performed together at the Hollywood Bowl in Los Angeles in September 1980, and 40 years since they last performed on stage in the UK.
In a press release issued ahead of the conference, the comedy troupe promised to perform "some of Monty Python's greatest hits, with modern, topical, Pythonesque twists".
Ticket prices
Warwick Davis, who recently starred in the Monty Python-inspired musical Spamalot, introduced the "five legends" at the London's Playhouse theatre, claiming the venue for the reunion had been decided after a series of bids.
Opening a series of gold envelopes, he claimed the winner was first, Qatar, then Meryl Streep.
Finally, he announced the show would take place in London at the O2 Arena on the 1 July, with tickets going on sale on 25 November.
The top price for tickets will be £95 and the lowest will be £26.50 -"only £300 cheaper than the Stones", quipped Idle.
Cleese said they would almost certainly include popular sketches such as The Crunchy Frog and The Dead Parrot, but reiterated that there would also be "some new material".
"People do really want to see the old hits but we don't want to do them in a predictable way," he said.
"The main danger we have is that the audience know the scripts better than we do."
Idle said they would also include some material that had never previously been performed live.
He said it would be "a big show" likening it to "a huge musical", with input from choreographer Arlene Phillips.
He added that the event would be released on DVD: "We'll be filming it and we'll try to flog it later."
"It's more than just a performance - people enjoy the experience of performing with us," said Cleese.
The 74-year-old said "at first" the gig would be "a one and only" but refused to rule out further performances.
"The problem is getting us all together in one room," he added, citing Gilliam's multiple film commitments.
When they did eventually get together, Palin said the five of them just "still enjoy getting together to be very silly", adding that it was easier to do "now they are in their 70s".
"Silliness is always funny," added Jones.
"After you turn 70, you can be absolutely shameless," echoed Gilliam.
News of the reunion leaked in the press earlier this week, with Jones telling the BBC he was "quite excited".
"I hope it makes us a lot of money," he said on Tuesday. "I hope to be able to pay off my mortgage!"
Devoted fans
The last time the five remaining members of the comedy group appeared together in public was in 2009 at their 40th anniversary celebrations in New York.
The sixth member of the comedy troupe, Graham Chapman, died in 1989.
The press conference took place at the home of the Monty Python-inspired musical Spamalot - London's Playhouse Theatre.
Earlier this year, a film producer won a High Court case against the surviving members of Monty Python over royalty rights to the hit stage show.
Mark Forstater, who produced the 1975 film Monty Python and the Holy Grail, claimed he was underpaid royalties since the musical's launch in 2005. He estimated he was entitled to more than £200,000.
The six members of the Python team got to know each other firstly through university, and later through their work on television comedy programmes, including The Frost Report.
Monty Python's Flying Circus was made for TV between 1969 and 1974 and featured sketches and songs that fans can recite by heart.
The team went on to make films including Monty Python and the Holy Grail and Monty Python's Life of Brian (1979).
The well-known Life of Brian song, Always Look on the Bright Side of Life, was performed by Eric Idle at the 2012 Olympics closing ceremony.
Source: BBC
