Reagan National Airport turned 75 this month with much fanfare: a white-tablecloth luncheon. A historic flight. Giveaways and music.

But the biggest gift of all may be for harried fliers: a $1 billion renovation that will include a new concourse for short-hop flights.

More than 23 million passengers used National last year, making it the second-busiest airport in the region, after Baltimore-Washington International Marshall Airport. Last year was its sixth straight year of record-high passenger traffic. And there is little indication it will taper off.

And that’s the problem.

Such phenomenal growth was not part of the original plan for the airport, which sits on 860 acres of infill in the Potomac River. But in the years since President Franklin D. Roosevelt watched the first American Airlines DC-3 aircraft touch down on the runway, the airport has grown in ways that few could have anticipated. It now serves more passengers than Dulles International — an airport 14 times its size.

Passengers are on their way to the gates inside Terminal B at Ronald Reagan National Airport. The airport is facing challenges due to the need to accommodate increased use, baggage and security. (Astrid Riecken/FOR THE WASHINGTON POST)

The irony is that while airport officials have fought against growth at National, they have been forced to accommodate it. They have reconfigured space, even repurposing storage closets and conference rooms to manage the passenger boom. In 2014, they spent $37 million to remake historic Terminal A, adding security lanes and brightening the interior.

This fall, they will break ground on the airport’s biggest building project in nearly two decades. The centerpiece of the ambitious $1 billion program will be a new commuter concourse on the airport’s north side. The building will replace Gate 35X — a notorious choke point where travelers, in rain, sun or snow, are required to board shuttle buses to get to their planes.

Airport officials have been careful not to call the project an expansion, noting that they are simply putting a building around an outdoor space already in use.

“This capital program is not about growth,” said Margaret McKeough, chief operating officer at the Metropolitan Washington Airports Authority (MWAA), which manages National and Dulles International airports. “This is truly about relieving congestion. Travelers using Reagan National should expect a certain level of service. It is simply not acceptable to rely on the busing operation.”

The project has long been on National’s list of needed improvements but has been put off for years, in part because it was difficult to get airlines to agree on a plan and also to determine how to pay for it. In the late 1990s, then-USAir announced plans to build a $16.2 million commuter terminal at the airport, but 15 months after winning approval, the airline scrapped the project because of financing difficulties. This new effort will be paid for by the airlines, not taxpayers.

Passengers crowd a waiting room of Washington National Airport as airlines braced for holiday departures on Nov. 24, 1955. (Bob Burchette/The Washington Post)

While other airport officials might cheer such robust growth in passenger traffic, it has become a sore point at National, because that growth has come largely at the expense of Dulles. National’s close-in location and easy access via Metro has made it the airport of choice for D.C-area power brokers and residents who do not want to make the more-than-20-mile road trip to Dulles.

The growth in passenger traffic combined with necessary changes in aircraft flight patterns also has led to a rise in noise complaints.

Ed Solomon of the D.C. Fair Skies Coalition, an anti-noise group formed to demand changes to flight paths, said his group opposes any new building at National. “Dulles is where we have to grow,” said Solomon, the group’s chairman. “We’re very concerned about all these incremental increases in infrastructure at National.”

National’s airport manager said the math explains it all. National was built to “comfortably” handle 15 million to 16 million passengers a year but is now serving more than 23 million, said Paul Malandrino Jr., the airport manager.

By contrast, Dulles has struggled to attract travelers. Between 2011 and 2013, Dulles lost nearly 200,000 seats to National, MWAA officials say. This year, the Virginia General Assembly approved a plan to spend $50 million over two years to help Dulles attract new airlines and more passengers. Two years ago, the airlines and the MWAA also signed an agreement that allows revenue sharing between the two airports. The agreement also would require airlines to compensate the MWAA if Congress opens the door to more long-distance flights to and from National.

National’s growth has made it the envy of other airports across the country that are struggling to attract airlines and passengers, said Kevin Burke, executive director of the Airports Council International-North America.

“A lot of airports don’t have the financing available to do what MWAA is doing,” he said. “Airlines and passengers want to be there. That said, they do have a capacity problem.”

Roger Natsuhara, vice president for engineering at the MWAA, said that work on the new terminal will begin this fall but that passengers will not begin to see changes until the spring. The project, which will include road improvements and a new parking garage, is expected to be completed in 2021.

Since some of the work will require blocking traffic lanes, much of it will be done at night. Crews will have to pack up their equipment after each shift to allow for traffic flow — not the most efficient way to work, but the only way to ensure smooth flow during National’s busiest hours.

There is no denying that the project will bring much-needed relief for 1.2 million travelers who pass through Gate 35X each year.

For years, the MWAA has used buses to move passengers from Terminal C to one of 14 boarding areas on the north end of the airport. Fliers must ride an escalator down to a small holding area before being ushered through one of five doors to catch buses that ferry them to their planes. On the social networking app Foursquare, one commenter called it “The absolute most confusing and ridiculous gate, possibly in the world.”

The MWAA’s executive offices and two hangars will be leveled to make room for the new concourse. Airport officials declined to say how large the structure will be, noting only that the square footage of the terminal complex will increase “by less than 5 percent.”

The project also will include changes to National Hall, the main, glass-enclosed walkway on the concourse level of Terminals B and C that features shops and restaurants. Security screening will be moved upstairs to the airport’s arrival level.

The shift will close off popular eating spots such as Legal Sea Foods and Ben’s Chili Bowl to the public, but airport officials think the change will boost revenue because passengers will have more time — and more places to eat and shop once they pass through security. Concessionaires have long complained that they lose out on revenue because passengers cannot linger since they must pass through security.

The last major building project at National was the addition of Terminals B and C, which opened with fanfare in July 1997. The 1.1 million-square-foot addition was designed by César Pelli, who once worked for Eero Saarinen, the Finnish American architect who designed the terminal at Dulles Airport. As the cost of the project at National escalated, it drew criticism from airline officials and members of Congress, who scoffed at the design, which included 74 “Jeffersonian” domes. Ultimately, officials cut costs by reducing the size of the building and reducing the number of domes by 10. In the end, the project cost $450 million and opened at least three years behind schedule.

On Thursday, in the historic section of Terminal A (now closed to passenger traffic because of security concerns), aviation luminaries raised glasses to toast the airport’s milestone birthday. Outside, Flagship Detroit — a restored American Airlines DC-3 aircraft that was the same model propeller plane that landed at National when it opened in 1941 — sat on the ramp.

“Reagan National is not just an airport, it’s a national treasure,” said Doug Parker, chief executive of American Airlines. “It’s at the epicenter of so much of what happens in this country.”