WASHINGTON (AP) _ The Pentagon announced Monday it had assigned the military designation ''X- 31'' to an experimental aircraft program designed to explore ways of increasing the maneuverability of jet fighters.

The Pentagon kicked off the program last year when the Defense Advanced Research Projects Agency, or DARPA, approved a $70 million development effort involving U.S. and West German companies.

The X-31 is the first experimental aircraft program that the United States has undertaken in cooperation with a foreign country. The Rockwell International Corp., the General Electric Co. and the German firm of Messerschmitt-Bolkow-Blohm are the prime partners in the effort, which calls for the production of two prototype planes.

The idea, according to the Pentagon, is to explore the use of new technologies that could ''produce dramatic improvements in survivability through increased maneuverability for fighter aircraft during close-in aerial combat.''

Among other things, the Pentagon has said it would like to explore ways of breaking the ''stall barrier'' by building a plane that could pursue high angles of attack or other maneuvers that normally reduce air flow over the wings and cause mid-air stalling.

Among the technologies being explored are the use of ''vectored thrust'' engines; better aircraft control systems, and improved protection against G- forces for air crews.

DARPA began the program on behalf of the Naval Air Systems Command and the prototype planes will be tested at the Naval Air Test Center in Patuxent River, Md. The first flight tests are expected in 1989.