An Air Force general told reporters in Great Falls, Mont., that he is pleased with the progress of the radar-dodging Stealth fighter, a plane so secret it officially does not exist. "I've visited the factory. I've seen the airplane," said Gen. John T. Chain, commander of the Strategic Air Command, mentioning the Stealth by name at a news conference. Unlike the better-known Stealth bomber, the experimental fighter is a subject about which military officials have been close-mouthed. They imposed strict secrecy in July after the crash of what was believed to be a Stealth fighter near Bakersfield, Calif. The pilot was killed.