For ex-soldier "Ricky Bobby," who says riding a motorcycle is as good as Prozac, the annual Rolling Thunder rally in Washington over the Memorial Day weekend is an unmissable ride.

On this sunny Sunday, tens of thousands of leather-clad, tattooed veterans descended on the US capital on Harley Davidson motorcycles, as they have each year since 1988.

The original group of bikers, all of them Vietnam War veterans, decided to ride into Washington each year on or around the Memorial Day holiday to demand the return of remains of prisoners of war (POWs) and those missing in action (MIAs).

"We come here first to honor the veterans," stressed Ricky Bobby, who declined to give his real name.

But, he conceded, "we spend good time with our friends too."

The ex-Marine was deployed to Kuwait during Operation Desert Storm in 1991. The 43-year-old, who served 22 years before retiring, rode his Harley 435 miles (700 kilometers) from North Carolina to attend this year's sunny parade across the Memorial Bridge that connects Arlington National Cemetery to Washington.

The name of the rally refers to Operation Rolling Thunder, an aerial bombardment campaign launched against North Vietnam by then US president Lyndon B. Johnson in 1965.

There are an estimated 1,500 POWs and MIAs from the Vietnam war whose remains were never recovered.

But, while its roots come from fighting for POWs, over time, the Rolling Thunder parade has become a boisterous display of unabashed patriotism and a tribute to veterans of all foreign wars.

"At the beginning, Rolling Thunder was created by Vietnam's veterans, and today other veterans from other recent wars keep on," Ricky Bobby said.

He said riding is the ultimate form of relaxation for him, and he can understand why so many other ex-soldiers have also taken up the hobby.

"It's freedom -- an expression of freedom. It's the best Prozac, you just relax on the road," he said, comparing the feeling to the well-known antidepressant.

Another biker, who goes by the moniker "Long Island," agreed: "I can be very upset, very, very upset. Then I ride my bike, and I'm very calm."

As his nickname implies, the 65-year-old hails from New York. The Vietnam vet, who sports a long white beard, recalled that when he returned from war in the 1970s, veterans didn't get the same kind of recognition they do now.

"Some were considering us like baby killers or things like that," he remembered.

"It took some time, but now it's really better. And between us, there is a real camaraderie. We've very close," he added over the roar of thousands of spectators cheering for the bikers on parade.