Boeing Co. has signed an agreement with International Bank to buy 34 acres near Tysons Corner for construction of two new buildings which will house part of Boeing's computer subsidiary, the Washington Post has learned.

Officials at both Boeing and the bank were reluctant to discuss the deal yesterday, pending the filing soon of a site plan for the project with Fairfax County officials.

But one source familiar with Boeing's plans said, "At this point, it's definite. The only thing that will stop the project is site plan approval."

According to several sources, the project involves both a consolidation and an expansion for Boeing Computer Services, Inc., a wholly owned subsidiary of the Seattle-based aircraft manufacturer. And it represents another commercial plum for Fairfax County which in recent months has witnessed what some planners say is the beginning of a rush of major firms to the area. Among those moving there so far are Mobil Oil. AT&T's Long Lines divison and United Nuclear Corp.

Boeing currently has about 300 employees in Fairfax, located in two buildings in McLean and one building in Falls Church. The space in all three buildings is leased. One of the leases expires in the fall of 1979, and Boeing has no option to renew.

The computer subsidiary, which has its national headquarters in Morristown, N.J., has been scouting the Washington area for months for a place to consolidate its local operations. These include the company's eastern district headquarters, its Washington sales office, a data center and a space and military applications center.

The site Boeing has agreed to buy from International Bank is reportedly located southeast of the Tysons Corner shopping center and is situated across the beltway from a Ramada by Rt. 7 and on the west by Gallows Inn. The land is bordered on the southeast by Rt. 495, on the northeast Road. International Bank is a diverse financial services company based in Washington.

Sources said Boeing initially plans to put two buildings on the site, one of about 75,000 square feet to house computers, the other of about 80,000 square feet for office personnel. The cost of the land and the buildings was pegged by one reliable source at $15 million.

"The project is designed to better than double the company's computer capacity," a knowledgeable source said. The new complex, the source added, will likely employ 400, about 100 more than now work for Boeing in Falls Church and McLean.

But that's only phase one. "We were told they'd eventually probably bring in about 500 to 600 employees," said Samuel Finz, deputy county executive in Fairfax "We're obviously very pleased by it all."

Finz said the site currently is zoned for industrial use and will have to be rezoned for Boeing. He said the county's planners must wait to receive a formal copy of Boeing's plans before they can say anything further about the project.