The museum for black America: a force for unity – or division?

To paraphrase Martin Luther King – here at last, here at last, here at last. Almost a century after the idea was first mooted by black veterans of the Civil War and almost half a century after the leader of the civil rights movement delivered his immortal "I have a Dream" speech, an African American museum is finally about to rise in the heart of Washington.

The National Mall, stretching from the US Capitol to the steps of the white marble Lincoln Memorial where King spoke in 1963, is perhaps the capital city's most special place, a two-mile vista lined by monuments to the country's greatest leaders and heroes, and by wonderful museums celebrating America's history and achievements. But there have been some notable omissions.

Last year one was corrected with the dedication of a memorial to King – even though the event has been somewhat marred by a controversy over the inscription on his statue, a version of lines from one of his final sermons that, according to the poet Maya Angelou, makes King sound "like an arrogant twit".

And on Wednesday another omission was corrected. In a moment of perfect historical symmetry, Barack Obama, America's first black President, led the ceremony for the future National Museum of African American History and Culture, devoted to the miseries, tribulations and triumphs of black America, scheduled to open its doors in late 2015.

The site is one of the last available on the ever more cluttered Mall. But it is also one of the best: close to the Washington Monument and across the street from the recently re-opened Museum of American History.

The long and laboured story of the 19th member of the Smithsonian complex has itself been a small slice of US history. The idea of a memorial to African Americans on America's national space was first put forward in 1915 by a group of black Civil War veterans, but went nowhere. Then in 1929, Congress did approve such a project, but failed to provide funding.

After the great civil-rights breakthroughs of the 1960s, pressure grew for a full-scale museum. Again though, nothing happened – not least because of the ferocious opposition of the late Jesse Helms, the powerful Senator from North Carolina and a last bastion in Washington of the bigoted "Old South", who insisted that a museum was "unnecessary", and a waste of taxpayers' money.

At this point, one may wonder how it was that the Holocaust Museum here – on the sacred National Mall – was approved and completed in barely a decade. And it deals with an event that does not belong to the US national experience.

The Holocaust Museum opened in 1993. In 2004 came the National Museum of the American Indian. But only now is work starting on a national museum that tackles head on, among other things, the original sin of the United States. As its director Lonnie Bunch told The Washington Post, slavery was central in shaping US history, but to this day remains "the last great unmentionable" in public discourse. "We will examine the dark corners of the American experience in a way we're not always comfortable in doing."

More than 100 black history museums already exist across the country, including the National Civil Rights Museum at the former Lorraine Motel in Memphis, Tennessee, where Dr King was assassinated in 1968. Others are reminders of the huge impact blacks had had on American, and indeed global, culture – like the small house on the west side of Detroit where Berry Gordy set up his recording studio called Hitsville USA, now the Motown museum. In Washington, all these strands will be meshed together.

The $500m (£318m) project is being funded by $250m of private donations, and a similar amount from the federal budget. Already Mr Bunch has amassed 25,000 artefacts, spanning the entire range of African American experience, from slavery to civil rights, to their immense contributions in art, sport, music and almost every walk of national life.

There will be a detailed examination of slavery which, he says, may well focus on the miseries of individual people: their families, the slave auctions where they were bought and sold, and the plantations where they worked.

The African American museum will celebrate heroes of emancipation like Harriet Tubman (Mr Bunch has acquired the shawl given to Ms Tubman by Queen Victoria, which she wore two days before she died in 1913). There will be items that lift the heart – alongside exhibits on lynchings, and other brutalities and indignities perpetrated on blacks for much of the last century.

Visitors will see Louis Armstrong's trumpet, but also the original pine coffin that contained the body of Emmett Till, the 14-year-old boy bestially murdered in 1955 in Mississippi because he was reported to have wolf-whistled at a white woman. The coffin was deliberately left open at Till's funeral in Chicago by his mother, so the world could see the atrocities visited upon her son. The public outrage that followed gave powerful fresh impetus to the civil rights movement.

Today no one would dispute that the story of black America, for better and worse, is part of the story of all America. Even so, some see the new museum as part of a worrying trend, the subtle atomisation of a country that has always prided itself on being a sum greater than its disparate parts. Now, they worry, it's not so much E Pluribus Unum, ("out of many one" – the phrase on the Seal of the United States), but Ex uno plura ("out of one, many").

As the Virginia Congressman Jim Moran puts it: "I don't want a situation where whites go to the original museums, African Americans go to the African American museum, Indians to the Indian museum and Hispanics to the Latino American museum. That's not America."