Which spacecraft was used in the first successful American manned spaceflight?

The Mercury program was the first American manned spaceflight initiative, aimed at exploring human space travel. The primary objective was to place a human in orbit around the Earth and return them safely. The mission that achieved this was Mercury-Redstone 3, famously known as Freedom 7, which launched astronaut Alan Shepard on May 5, 1961. This flight marked a significant milestone in the space race and demonstrated crucial technologies for future manned missions.

In contrast, the Gemini program, which followed Mercury, aimed at advancing space travel techniques and included longer-duration flights, while the Apollo program focused on landing humans on the Moon. The Space Shuttle program further advanced human spaceflight capabilities but started many years after the first successful Mercury flight. Therefore, Mercury is correctly identified as the spacecraft used in the first successful American manned spaceflight.