The F/A-18 Hornet is actually a decendent of the F-16 Fighting Falcon. In the 1970's the Air Force wished to develop a fighter that was smaller and cheaper than the F-15 Eagle developed a few years earlier, and that could be exported to foreign allies. Northrop's entry into the competition was the YF-17, a dual engine, dual tailed fighter. In the end the YF-16 designed by General Dynamics (now owned by Lockheed Martin) won the competition, but many pilots still preferred the YF-17.
In a joint effort by Northrop and McDonnel Douglas the YF-17 was transformed into the F/A-18. The Navy preferred the dual engines which were safer for long missions over water. It also served as a small manueverable complement to the F-14 Tomcat.