LA JUNTA, Colo. (AP) _ A B-1B Air Force plane with six crew members on board crashed today during a training mission for the nation's newest bomber, but three crew members parachuted to safety, police said.

The other three crew members were unaccounted for, officials said.

An eyewitness, Bud Quick of La Junta, said he ''felt a thud,'' saw a ball of fire flare up behind some hills and saw three parachutes floating to the ground.

Todd Quick, also of La Junta, said he saw the bomber fly over and that it appeared to be in trouble moments before it went down.

Greg Ricken, who lives 20 miles east of La Junta, said the plane flew almost directly him and the tail appeared to be on fire. He said the smoke got heavier and the plane seemed to angle to the right before it went over the horizon and a fireball appeared.

The crash was the first ever for a regular production model of the B1-B, although a prototype of the bomber crashed in August 1984 in the Mojave Desert of California. One man was killed and two others seriously injured in that accident.

The bomber, attached to the 96th Bombardmen Wing, took off from Dyess Air Force Base in Abilene, Texas, early this morning, bound for the Strategic Training Range Complex near La Junta, about 60 miles southeast of Pueblo, the Strategic Air Command said.

The plane went down on the prairie four miles southeast of La Junta at 9:34 a.m., said Capt. Dave Thurston of SAC headquarters in Omaha, Neb.

He said he did not have any reports about the three crew members who police said parachuted to safety.

The plane was not carrying any bombs, the Air Force said.

Peterson Air Force Base in Colorado Springs has personnel en route to the crash scene, a spokesman said.

The B-1B, a strategic bomber and missile platform, is manufactured by Rockwell International's North American Aerospace Operations in El Segundo, Calif. It has a maximum speed of about 1,000 mph and a range of more than 7,000 miles when fully fueled.

The B-1B is the nation's newest long-range strategic bomber - the first such plane produced in more than 25 years. The first of the 100 B-1B's being purchased by the Air Force were activated in October 1986 at Dyess.

More than 70 of the planes have been delivered so far.

The B-1B is powered by four jet engines and is smaller in physical size than the B-52 but capable of carrying more bombs. It already holds a number of world aviation speed records for a plane of its size.

The B-1B and the so-called ''Stealth'' bomber - which has yet to enter production - are key components of the Reagan administration's military build- up. Both planes are designed to improve the Air Force's ability to penetrate Soviet border defenses in the event of a war.

The B-1B is designed to fly at almost the speed of sound at altitudes of only 200 feet above the ground.

In the first B1-B crash, of the prototype, the Air Force ultimately determined that crash resulted from the failure of the crew to properly balance the fuel load aboard the plane. The controls for that fuel-balancing process were changed before the B-1B went into production.

The Air Force has been criticized over the past year for its handling of the B-1B program, primarily because of acknowledged problems in developing the bomber's radar-jamming gear. The service, while conceding the so-called electronic-countermeasures equipment has yet to perform as designed, has nonetheless insisted the plane is already the best long-range bomber in the world.

The Air Force said today it was unclear at what altitude the bomber was flying when trouble developed aboard the plane. At present, as the Air Force trains its crews and breaks in the new plane, flights have generally been restricted to a minimum altitude of 500 feet.