Which aircraft was the first to break the sound barrier?

The X-1 aircraft was the first to successfully break the sound barrier on October 14, 1947. Piloted by Chuck Yeager, the X-1 achieved this milestone by reaching a speed of Mach 1.06, or 700 miles per hour, at an altitude of 43,000 feet. Its design specifically incorporated features aimed at supersonic flight, including a rocket-powered engine and a streamlined fuselage that minimized drag.

In contrast, the other aircraft listed were not designed or utilized for the purpose of breaking the sound barrier. The B-17 Flying Fortress was a heavy bomber used during World War II, primarily for strategic bombing missions. The F-86 Sabre was a jet fighter that operated in the Korean War and could reach near-supersonic speeds, but it came after the X-1 and did not initiate the supersonic flight era itself. The A-10 Thunderbolt II, designed for close air support, is a subsonic aircraft and was never intended for or capable of supersonic flight.

Therefore, the X-1's pioneering achievement in supersonic flight is a landmark moment in aviation history, making it the correct answer.