The planes, both from the United States Air Force's 69th Expeditionary Bomb Squadron, flew an 18-hour, 11,000 km mission from Andersen Air Force Base on Guam to take part in the commemoration of the 50th anniversary of the Battle of Long Tan at the Australian War Memorial.

Two of the squadron's pilots flew to Canberra to act as liaison officers.

"It was a privilege to take part in the ceremony," said one, who cannot be named for security reasons, but goes by the call sign "Slam", told Fairfax Media.

The 69th, which is normally housed at Minot Air Force Base in North Dakota, rotated to Andersen in February and March this year. "Deployments are for six months so we are coming to the end of our tour," Slam said.

The two planes that flew over Canberra in company with a Hercules, a Caribou, two DC3s, an Iroquois helicopter, a Sioux helicopter a Cessna 0-2 and Cessna Bird Dog, were B-52 H variants.

These, the last of a line that first saw the light of the day in 1952 and went into service in 1955, are 12.4 metres tall and weigh more than 83 tonnes when empty.

They are 48.5 metres long and 56.4 metres from wing tip to wing tip.

A B-52 H can carry up to 32 tonnes of munitions and has a maximum speed of 1047 km/h. The planes, the largest combat aircraft operated by the USAF, have a 16,000 km range.

Ian Thompson, president of the Vietnam Veterans and Veterans Federation of the ACT and the organiser of the Long Tan commemoration fly past, said he was surprised and delighted on Wednesday when told that not one but two of the big bombers were coming.

"We had asked for one," he said. "It meant so much to the USAF and the US government to have an aerial presence over Canberra for this event that they sent two planes just to make sure.

"Everybody we worked with on this, from the bottom to the top of the US government, was determined to make this a success. I believe this is in recognition of Australia's role as America's principal ally [in the Vietnam war].

"The USAF would have treated this as a high-profile training exercise and you wouldn't have gotten change out of a couple of million dollars."

Mr Thompson, a helicopter pilot in Vietnam who went on to fly fighters after the war before becoming a Qantas pilot, said the Long Tan anniversary had been a red letter occasion for his members.

"Nearly 60,000 men and women served in Vietnam," he said. "The [community] attitude towards is in 1974 and 1975 was abysmal. It has taken a long time to get over that slight.

"We had 100 veterans from the battle here today and 1500 or more Vietnam veterans all up. This is two to three times what you would normally expect at one of these events."