Installing a pedestrian bridge that will for the first time connect the two main parts of the National World War II Museum marks a major step in the completion of an ambitious $325 million capital expansion program, officials said Sunday.

As the steel frame of the 87-foot-long, approximately 20-ton American Spirit Bridge was hoisted off a flatbed truck, rotated 90 degrees and installed by two large cranes, museum officials marveled Sunday morning at the progress of a museum that has been under constant construction for more than five years.

“This is one more step in the completion of our mission,” said Bob Farnsworth, senior vice president of capital programs, at the site of what is slated to become a new pedestrian-friendly plaza underneath the bridge on Andrew Higgins Drive.

“It’s really going to function well for business and for visitors, and will be much safer,” he said. “It’s always been an issue getting people across this street.”

The bridge over Andrew Higgins Drive will connect the museum’s Louisiana Memorial Pavilion, home to the original D-Day Museum on the downriver side of Andrew Higgins,?with the rest of the 6-acre campus on the upriver side, creating for the first time a “seamless visitor experience,” museum officials said. At present, pedestrians in one part of the museum have to go outside and cross the street to get to the other part.

The bridge, due to open in December, is designed to expand the museum literally and figuratively, officials said, providing a connection from a “replica Union Pacific train car” on the downriver side to exhibits that explore how the war was fought and won in Europe, northern Africa and the Pacific.

As visitors leave the train car, designed to simulate the experience of new recruits headed to boot camp somewhere in the United States, the American Spirit Bridge will provide the sensory experience of new soldiers being shipped overseas. A soundtrack will simulate sounds of waves against the side of a ship, the rhythmic beep of Morse code and the echoes of naval commands.

The elevated walkway will provide atmosphere and context for the nearby exhibits, Farnsworth said, including two exhibits in the “Campaigns of Courage: European and Pacific Theaters” pavilion, slated for completion this year. The “Road to Berlin: European Theater Galleries” opened in December, and the “Road to Tokyo: Pacific Theater Galleries” are due to open by the end of 2015.

The bridge also will connect to the exhibit “Merchant Marine: We Deliver the Goods.”

Farnsworth said he anticipates a rise in museum attendance with the completion of this year’s projects.

“Every time we open a new pavilion we pretty dramatically increase the level of visitation,” he said, adding that the museum has “continued to exceed” expectations set out in a marketing study conducted years ago.

Last year, museum patrons filled more than 200,000 hotel room nights in New Orleans, a number that officials expect to grow. And as contractors continue to renovate buildings and plazas in the area, they will improve the streets’ drainage, Farnsworth said.

“Of course, our mission is to tell the story of World War II,” he said. “But our secondary mission is to improve the economic impact in the city. This is a great source of pride for the city and the state.”

The bridge, which was 75 percent funded by the state and 25 percent funded by private donors, has been designed to fit in with the historical architecture of downtown New Orleans, Farnsworth added.

“If you’re going to put a bridge in a historic neighborhood, it needs to be treated architecturally in a way that’s meaningful,” he said of the steel structure, which will be fitted with glass panels to match the museum’s pavilions.

After the bridge is opened, along with the “Campaigns of Courage” exhibits, the museum will turn to completing the Liberation Pavilion, which will focus on postwar history and what the war means today.

Ultimately, the entire capital expansion project will quadruple the size of the original facility.

The construction hasn’t always been easy, said Chris Combs of C.M. Combs Construction. “It’s been challenging on many levels,” he said, adding that crews have gone to work at night, after visiting hours. “We are doing it in the midst of a fully functioning and operating museum.”

But officials have given themselves a tight deadline to get the work done, according to museum President and Chief Executive Officer Gordon “Nick” Mueller, so that surviving World War II veterans will be able to enjoy it.

The museum had planned to complete construction in 2012 but was derailed by Hurricane Katrina.

As of last year, about 1 million of the 16 million people who fought in the war were still alive. About 12,000 of those lived in Louisiana.

“We’re losing our veterans across America and across this state,” Mueller said last year. “It’s our goal to get this museum finished while there are still some World War II veterans alive.”