It has long been one of aviation's dreams to develop an air transport that could take off vertically like a helicopter yet have the payload, speed and range of a fixed-wing airplane.

The Defense Department is betting billions of dollars that it has found the answer in a joint venture of two big aerospace names, Bell Helicopter Textron and Boeing Vertol, a Philadelphia subsidiary of the Seattle Boeing Co.

They are in the midst of a $2.4 billion program to develop the JVX (joint services advanced vertical lift aircraft program), a twin-engine "tilt-rotor" that is supposed to carry 24 fully equipped Marines at least 50 miles from ship to shore and back again.

The payoff is a military order for almost 1,000 JVX tilt-rotors. The Marines alone are planning to pay $22 billion for more than 500 of the craft by the year 2000.

Bell and Boeing are also looking for a commercial market for their creation and see it as particularly valuable in providing acceptably quiet and efficient center-city to center-city flights. The JVX in a commercial configuration could carry 44 passengers and land on a helipad or short runway.

So Bell and Boeing brought a road show to Washington yesterday in the form of the XV15, an experimental aircraft that is the so-called "testbed" for many JVX concepts. The XV15 was built by Bell with help from the National Aeronautics and Space Administration and the Army.

About 100 people -- a cross section of military-industrial complex with a smattering of civilians -- gathered at Bolling Field to watch the XV15 hover, then land after a flight from Teterboro, N.J., with Bell President Jack Horner in the copilot's seat.

Technicians from Montgomery County took noise measurements. The county is studying a rotorcraft ordinance, and, for a rotorcraft, the XV15 seemed relatively quiet. It measured 98 decibels about 75 yards from the landing site.

Rep. James C. Wright Jr. (D-Tex.), the congressman from Fort Worth, where Bell is located, jumped enthusiastically on the bandwagon. "There is no doubt the JVX represents the next major breakthrough in aviation technology," he said.

Wright praised the "unsung value" of military production, which provides, he said, "the opportunity for forward-stretching technology that would not have been possible to do commercially."

George G. Troutman, Bell's vice president in Washington, compared the JVX potential for commercial production with that of the Air Force KC135 tanker that Boeing built, then turned into the 707, the first truly successful civil jet transport. The 707 fuselage is still around more than 20 years later, in various lengths, as part of the 727, 737 and 757.

The key technical development in the tilt-rotor system is the use of propellers as rotors when the aircraft needs to take off, land or hover. The XV15's turbine engines are located at the ends of the wings and rotate. The propellers can be pointed skyward or, once the plane is airborne, pointed straight forward. Then the XV15 looks like a twin-engine turboprob, with somewhat larger than normal propellers and the engines farther outboard than is usual. The JVX will use the same system, although it will be about 1 1/2 times as large and have more than twice the payload.

The JVX in a commercial configuration, Bell officials said, would have a range of 725 nautical miles and a cruising speed of 300 knots, which places it comfortably within the ranges of several commuter planes in production today.