Streetwise scenes with plenty of punch

You may have never heard of George Bellows – and understandably so.

After all, American artists and the groundbreaking achievements of early 20th-century painting rarely feature in the same sentence. If you have, it's probably because of his boxing paintings, scandalous at the time and which even a century later possess the visceral force of a right hook in the solar plexus.

But ignorance should prevail no longer. The first major Bellows retrospective in three decades opened a week ago at the National Gallery of Art in Washington DC. Simultaneously thrilling and draining, the exhibition stakes his claim as one of the most important and wide-ranging artists of his age, in America or anywhere else.

Since his early death in 1925, Bellows' reputation has fluctuated. At first he was hailed as a genius, but then he rather faded from view, considered a typically "American" painter. Never once in his life did he venture abroad. And what, too, could be more American than his very emergence as a painter – the strapping young man and gifted athlete who, legend has it, turned down an offer from the Cincinnati Reds baseball team before he dropped out of college in Ohio to pursue his first love at the New York School of Art?

Bellows is usually lumped in with the realist Ashcan School that flourished at the time, so named because of its preoccupation with New York's grimy human underside, of immigrants and tenement dwellers, day labourers and street urchins. Bellows rendered all of those, as a visual poet of the teeming post-Gilded Age metropolis. However, he was much else besides. In the exhibition, you start to grasp his place in art history, amid the transition from prim Victorian convention to the modern era, and the extent to which this supposedly all-American painter drew on centuries of European tradition.

Over the years, critics have accused Bellows of being crude and unsubtle, all energy and no nuance. And America's detractors might again say, what could be more American than that? But though his work vibrates with raw intensity, the foreign influences are unmissable. From the anticipatory expressionism of El Greco to the mysterious portraiture of Manet and Goya's horrific depictions of war, all are to be found in his work.

The Washington show brings together for the first time Bellows' three great boxing paintings – Club Night (1907), followed by A Stag at Sharkey's and Both Members of this Club in 1909. They are surely among the most powerful sports images ever committed to canvas, capturing the brutality of a then semi-legal sport that took place in private clubs, where rules were few to nonexistent. The fighters are streaks of flesh and blood, like meat in a butcher's shop. The grotesque faces of the rivetted crowd, receding into an enveloping darkness, could be lifted from Brueghel the Younger.

A year before he died, Bellows produced a last great boxing painting, which rounds off the Washington show, of Jack Dempsey as he was knocked out of the ring in his legendary 1923 title fight with Luis Angel Firpo, the "Wild Bull of the Pampas". By then both boxing and Bellows' style had changed. Prizefighting was now legal, and Dempsey and Firpo is better lit and almost stylised, a kind of poster-art. But right across the board, Bellows' output was moving in that direction.

If boxing provided the bookends to Bellows' career, what came in between was a kaleidoscope of genres. There is of course his Ashcan canon, including a celebrated quartet of paintings of the construction of Penn Station, shown in Washington together for the first time, and the 1913 Cliff Dwellers that depicts tenements as human anthills, a study in perpetual motion.

Bellows, whose socialist sympathies should now be obvious, also has a haunting eye for the especially vulnerable as they cope with life's inequities. His early work Kids (the word in those days was slang for streetwise poor children, not the generic term it is now) could illustrate the pages of a Dickens novel. So too could his 1908 portrait Paddy Flannigan, of a scrawny, bucktoothed boy, wise far beyond his years. Equally disturbing is New York, a bleak and claustrophobic winter cityscape of 1911, teeming with people, yet somehow inhuman. It is vintage Bellows, compelling yet exhausting in its refusal to offer a single quiet spot where the eye may rest.

His range is breathtaking. Throughout his career he produced portraits, not just of street kids and other ordinary people of New York, but of his family and friends. In the portraits the classical influences upon him are most vivid. Most of them share an unsettling intensity – none more so than Emma at the Piano, one of 11 portraits of the wife to whom Bellows was devoted. She turns directly to face the viewer, her blue eyes opened wide, challenging one to look away.

Then there are seascapes from his beloved Maine, rich experiments with colour, as well as winter landscapes contrasting pristine snow with urban grime, that make you feel cold looking at them. Bellows could paint not only the poor but also the rich, as they disported themselves at lawn tennis or polo, or lazed in Central Park on a verdant summer's day. Yet by 1918 he was depicting German war atrocities in Belgium with the vividness of a Goya. He was a lithographer and illustrator of astonishing power as well.

As he once put it, "There is nothing I do not want to know that has to do with art and life," and his topics, too, are as relevant as ever today: the inner city of course, and the widening gulf between rich and poor – but also immigrants, race, sport and hot-gospelling preachers (in Bellows' era one Billy Sunday, the Pat Robertson of his time).

In the end, one can only wonder where the restless drive of this supremely gifted artist would have taken him had he not been struck down by peritonitis at the age of only 42. The sheer variety of his output, and his readiness to experiment, suggest a man searching for his ideal means of expression. When he died, George Bellows was still a work in progress.

A smaller version of the Washington show is coming to the Royal Academy in London next year. But if you want the full Bellows experience, and you're in Washington DC this summer, then you must drop by the National Gallery of Art.

National Gallery of Art, Washington DC (nga.gov) to 8 October