On June 11, 1974, the Northrop YF-17 A 72-01569 became the first American fighter to break the sound barrier in level flight when not in afterburner.
In the early 1970s, the ‘Pentagon came up with the concept of a smaller and less expensive Lightweight Fighter Program. Eventually, the nod was given to General Dynamics to build two YF-16 demonstrators, while Northrop would build two YF-17s based on the P-530 (now called the P-600). The Air Force would fly the prototypes head to head and pick a winner.
Both the YF-16 and YF-17 performed well. The YF-17 became the first USAF aircraft to break the sound barrier in level flight without using afterburners. While the YF-17 outperformed the YF-16 in many areas, the YF-16 used a proven engine, it had a longer range and was less expensive. As a result, the YF-16 won the contest and went on to be produced in large numbers as the Fighting Falcon. The two YF-17 were sent to NASA, which used them for research for a few years before retiring the two prototypes.
That was not, however, the end of the story. The US Navy needed an airplane to replace the carrier-based F-4 Phantom II, A-6 Intruder, and A-7 Corsair II. When the design program floundered, the main competitors were ordered to look at the aircraft from the Lightweight Fighter Program. The Navy preferred two engines when flying over open water, so the F-16 was not a choice. Northrop determined that they could adapt the YF-17 design to naval use by refining the design and beefing up the landing gear. Northrop teamed up with McDonnell Douglas and submitted a proposal to the US Navy. The proposal was accepted, and the F-18 program was born.'(1)
(1) Source: https://www.johnweeks.com/yf17/index.html