Two Northern Virginia computer companies yesterday won $15 million-plus contracts each with the U.S. Army to compete against each other in developing something much on the minds of the military right now: a computerized system to improve mobilization of Army and National Guard reserves.

The system, which will not be completed until 1996, is intended to greatly speed up the kind of work the military faces currently in the massive buildup of U.S. forces in the Persian Gulf.

Ultimately, the project could be worth about $1 billion over the 12-year life of the contract.

The companies, Computer Sciences Corp. of Falls Church and Boeing Computer Services of Vienna, each will spend the next year developing prototype systems. Each is working with a host of subcontracting companies, most from the Washington area.

Real-life reservists will use the two systems in a runoff next spring to help determine the final winner.

Company officials said the new system would greatly modernize the existing hodgepodge of computer punch cards and other older technologies that hold information about the readiness of thousands of military units around the country.

The computers will be placed at more than 4,700 locations around the country and abroad, linking nearly 10,000 reserve units and giving Army officials up-to-date information about the status of each.

Boeing Computer Services is working with Atlantic Research Corp. of Rockville, Contel Federal Systems ASC of Chantilly, Digicon Corp. of Bethesda, General Research Corp. of Vienna, Innovative Logistics Technologies of McLean, Presidio Corp. of Suitland, Science Applications International Corp. of McLean and Titan Applications of Vienna.