WASHINGTON -- The Pentagon has begun a search for a replacement for Air Force One, but it will probably be some future president who first uses the new presidential airplane.

The Air Force advertised the project to defense contractors this month, saying the first plane should be ready in 2017.

The Air Force would buy two more over the next few years, providing a fleet of three jumbo jets to replace the current fleet of two modified Boeing 747s.

The presidential 747, at five stories high and 4,000 square feet of space inside, is impressive and comfortable. But the 747 is an old design, and commercial versions are being retired from U.S. passenger service. Parts are getting more expensive and harder to come by, the Air Force said in its Jan. 7 solicitation.

The service concluded that it would be cheaper in the long run to buy new planes. It plans to use a souped-up version of a commercial plane, as it did with the 747s.

The current powder blue and white jets were delivered in 1990 and were expected to last about 30 years.

Even if President Barack Obama serves two terms he will be out of office in January of that year. The Pentagon counts in fiscal years, which begin in October, so if the plane is delivered in the first quarter of 2017 a second-term Obama might get a ride.