Eleven years on, they have raised £200,000 for charity and have more than 600 members across the UK.
On Friday, Boobs and Brass was given the BBC Music Day Brass Band Award, for the band that best embodies the BBC Music Day 2017 theme - "the power of music".
The band was born when brass musicians Jane Nichols and Maggie Betts decided to call on all the female players they knew for a charity concert.
With a 25-strong band, the friends expected to raise a few hundred pounds. In fact, they took £5,000.
"We were absolutely dumbfounded," says Mrs Betts, a 63-year-old cornet player from Northamptonshire.
That first show attracted a big crowd - some of whom, Mrs Betts suspects, were only there because they were curious about the name.
"I don't think they were quite sure what they were getting," she says. "They might have been disappointed!
"All the girls said, 'That was brilliant, we've got to do it again.' And it's just never stopped since."
The band has since grown and split into three branches covering the Midlands, north and south of England. More than 600 people aged between 12-73 are on the database, and the first international offshoot will launch in New Zealand later this year.
Before that inaugural concert, Mrs Nichols and Mrs Betts decided to choose a charity each and split the proceeds. Mrs Nichols gave her half to a campaign for equipment at Kettering Hospital, where she worked.
At the time, a fellow member of Mrs Betts's regular band had just been diagnosed with breast cancer. So she chose Breast Cancer Now.
Over the last 11 years, the Boobs and Brass band members - who are all unpaid - have continued to give concert takings to charity. The total stands at £206,000 - more than £170,000 of which has gone to Breast Cancer Now.
It is a cause that has become particularly personal to the musicians.
Mrs Betts says about 20 members have had treatment for breast cancer, and three are going through it now. The band has become a support group as well as a source of enjoyment.
"It's a very tight-knit community when you've got members who are going through something so drastic," she says. "You just want to be there for them. It's scary. It could be any one of us. You just never know whether it's going to be you."
The struggles of those band members have made the others especially keen to keep raising money, Mrs Betts says.
"We've had a couple of thoughts about how long this band is going to run. Perhaps we ought to call it a day. And the girls have said, 'No way, we've got to carry on.'
"You see a friend next to you wearing a wig and going through chemotherapy and she's still at Boobs and Brass playing - you think, we've got to do it for people like you. It makes you more determined."
When they started, the name Boobs and Brass "shook a few people" - and some people still find it distasteful, Mrs Betts says.
She tells any critics about the work they do and assures them they are "not just a load of silly girls dressing up and flashing our boobs".
But they have provided a refreshing change in the brass band world, which has traditionally been overwhelmingly male.
Mrs Betts was the first woman to be allowed to play in her local brass band in 1969. "It was quite a big thing," she says
And now, Boobs and Brass have relaxed their women-only rule.
Playing with them in the annual Whit Friday brass band contest in Saddleworth, Greater Manchester, on Friday, were two men.
"It's difficult to get lady bass players - it's a big, heavy instrument," Mrs Betts says.
"But they're also representing the men who get breast cancer. As long as they're brave enough to put the pink jacket on, they can be part of it.
"We don't like to bring too many [men] in. But the odd one here and then is acceptable."
The BBC Music Day Award was presented at the Whit Friday procession, and Mrs Betts says it's a reward for all the band's musicians and supporters.
"When you are given an award like this, that is our way of being able to say, 'Thank you girls, this is what people think of you. This is what you've done'.
"To us that is worth a million pounds. An award like this being offered to us is so very special."
Apart from raising more money for charity, Mrs Betts has one ambition left.
"I'm hoping that somebody will make a film of it some day," she says.
It's not beyond the realms of possibility - Brassed Off meets Calendar Girls, perhaps. A tale of real people breaking the established rules and winning the day with heart, ingenuity and generosity.