This, again, is a question that could have many answers depending on what your interpretation of the term 'UFO' is. We could go as far back as the Old Testament and, as some commentators have done, interpret what Ezekiel saw as a UFO. There are many historical accounts that leave open an interpretation of events that might today be described as UFOs but things first got relatively serious around the 1880s with reports of strange "airships". The so-called modern era of the UFO began on June 24, 1947 when a pilot by the name of Kenneth Arnold spotted 9 disk shaped objects over Mount Rainier close to Seattle. The following chronology of events might give you an idea of recent ufological history:
1.First report of a "large saucer" made by John Martin in Denison, Texas: 1878.
2. First reported unidentified "airship" sighting: March 29, 1880.
3. So-called "Foo Fighters" reported by pilots from both Allies and Axis forces during World War II.
4. Kenneth Arnold spots 9 disk shaped objects flying in a manner similar to rocks being skipped across the water: June 24, 1947.
5. Roswell Incident - Recovery of
what the 509th Bomb Group at Roswell Army Air Field first called a "flying
saucer" on an official press release: July 8, 1947.