It’s been 40 years since someone has built over the Mass. Pike. Until now

Developer Samuels and Associates has secured financing for a $700m office-and-hotel complex, and work is expected to start next week.

An artist's rendering of the lower levels of the $700 million complex that developers plan to build over the Massachusetts Turnpike in the Back Bay.Elkus Manfredi

For four decades, building atop the Massachusetts Turnpike has been the great white whale of Boston development. Full of promise, but no one could quite pull it off.

Now it’s finally about to happen.

Developer Samuels & Associates closed Friday on financing for a $700 million office-and-hotel complex over the Pike along Massachusetts Avenue. Construction is expected start next week.

It’s a huge project, one that over the next three years will reshape the ragged western edge of the Back Bay and provide a new front door to drivers entering downtown Boston from that direction. The complex undertaking will require construction of a deck above eight lanes of highway and busy rail lines. It’s also happening amid an enormous economic crisis that no one saw coming. That required some last-minute financial juggling, said Samuels chairman Steve Samuels.

“We’re really excited to be moving this forward,” he said. “It’s a really complicated environment.”

The complex will run along Mass. Ave. between Boylston and Newbury streets. It includes a 20-story office tower — about half of which has been leased to online auto marketplace CarGurus — on the corner of Boylston, and a 13-story Citizen M hotel on the corner of Newbury. Between them would be a half-acre public plaza with retail space, improved sidewalks, and a new entrance to the Hynes Green Line stop. The westbound onramp to the Pike will move off of Mass. Ave. to Newbury Street.

Planning has been underway since 2013, when the Massachusetts Department of Transportation awarded the site — known as Parcel 12 — to Samuels. It’s one of three so-called air rights parcels over the Pike near the intersection of Mass. Ave. and Boylston Street that MassDOT awarded to developers.

Of those other projects, one fell apart shortly before breaking ground last summer, leading to an ugly lawsuit between construction magnate John Fish and veteran developer Steve Weiner, while the other has remained dormant for years. Several other projects — from the ill-fated Columbus Center to a skyscraper alongside Copley Place — have fizzled, largely because of the engineering and economic challenges involved in building over the Pike. Not since Copley Place opened in the early 1980s has a large project come to fruition.

Samuels said the new project will better link Back Bay and the Fenway, where his firm has significantly remade Boylston Street over the last decade. It will also change the way that pedestrians and cyclists experience Mass. Ave., making a windblown, auto-centric strip of the city into something far more pleasant.

That will be most welcome, said Meg Mainzer-Cohen, president of the Back Bay Association, who said the project promises to create a new feel for a key stretch of the neighborhood.

“Now it’s not even really a bridge. It’s just this deck over the Pike. You’re looking at a bus shelter and the highway,” she said. “Now you’re going to see a hotel, an office building, and a park in between the two.”

But it will take some doing.

Samuels and MassDOT have spent months planning for construction of a deck above the highway and Commuter Rail lines, which will take about a year. That will probably involve frequent lane closures on the Turnpike — Samuels said they’ll release their traffic plan within a few weeks — at least until the deck is finished and construction of the actual buildings begins above it.

Transportation Secretary Stephanie Pollack said her agency is glad to see the project moving forward, and is designing plans that aim to keep traffic and trains flowing during construction.

“MassDOT is pleased that Samuels & Associates will be building a mixed-use air rights development with multi-modal transit benefits over the Massachusetts Turnpike,” she said in a statement. “[We] will work closely with the developers and their construction team to minimize disruption to Mass. Pike commuters when construction begins to affect highway capacity in mid-August.”

A second major air rights project just up the road — the long-planned Fenway Center complex — is also on track to start work soon, said its lead developer, John Rosenthal, on Friday. The developers of the $1 billion project have money in hand to build a large deck over the Pike, he said, and are just a step behind Samuels, finalizing the same sort of agreement with MassDOT that their neighbor signed Friday. Rosenthal hopes to close on that arrangement in a few weeks and begin work soon after, enabling the two projects to coordinate lane closures on the Pike.

“We expect to start construction on our foundation and deck in August,” he said. “I’m still pinching myself.”

For Samuels, getting to the finish line involved a lot of juggling, especially after the coronavirus crisis hit in March. There were many conversations with investors and lenders, though Samuels said none bailed on the project. They’ve been closely monitoring the cost of construction materials and labor, which have shifted rapidly. And the anchor tenants — CarGurus and Citizen M, both of whom signed onto the project last year — remain committed, though CarGurus scaled back its initially announced lease for 275,000 square feet to 225,000.

“We continue to be excited about establishing our new headquarters at this prime Boston location, recognizing our move is still a few years away,” said CarGurus chief financial officer Jason Trevisan, in a statement. “This building gives us the opportunity to create a dynamic space for our employees that will enable continued innovation and business growth.”

Samuels still has about 210,000 square feet of office space to lease, in an office market that appears far softer than it did just a few months ago. The project also includes about 50,000 square feet of retail, another industry being shaken by the pandemic. Samuels said the company is likely to let things settle down for a few months before marketing that space.

The fact that Samuels & Associates was able to raise money now to launch a $700 million project, the likes of which hasn’t been built in Boston in decades, is an enormous vote of confidence in the city, and the Back Bay in particular, Samuels said.

“Everybody’s making a long-term bet here,” he said. “On Boston, in particular on the Back Bay and the Fenway. And, yeah, I think that’s a pretty safe bet.”