As Arnold recalled, the afternoon was crystal clear, and he was cruising at an altitude of 9,200 feet. A minute or two after noting a DC-4 about 15 miles behind and to the left of him, he was startled by something bright reflecting off his plane. At first he thought he had nearly hit another aircraft but as he looked off in the direction the light had come from, he saw nine “peculiar-looking” aircraft flying rapidly in formation toward Mount Rainier.
As these strange, tailless craft flew between his plane and Mount Rainier and then off toward distant Mount Adams, Arnold noted their remarkable speed — he later calculated that they were moving at around 1,700 mph — and said he got a pretty good look at their black silhouettes outlined against Rainier’s snowy peak. He later described them as saucer-like disks … something the gentlemen of the press glommed on to very quickly.
At the time, Arnold said, the appearance of these flying saucers didn’t particularly alarm him, because he assumed they were some kind of experimental military aircraft. If they were, nobody in the War Department (soon to be merged into the Department of Defense) was saying.
The official position of the Army Air Corps was that Arnold saw a mirage or was hallucinating. The term "flying saucer" received lots of publicity and many other reports rolled in shortly afterward. The incident marked the beginning of the UFO craze. Link
(Image credit: U.S. Air Force)