The longest distance ever traveled by an airplane depends on the context in which you consider "distance." There are a few different ways to measure it:
Single Flight: The longest non-stop flight in terms of distance was achieved by Singapore Airlines' Airbus A350-900ULR (Ultra Long Range) aircraft. It covered a distance of approximately 9,537 miles (15,344 kilometers) on the Singapore Airlines route between Singapore and Newark, New Jersey, USA. This route was temporarily suspended due to the COVID-19 pandemic but was the longest commercial flight available before the suspension.
Total Distance Traveled: The record for the greatest total distance traveled by an airplane is held by the Voyager aircraft, a specially designed aircraft for global circumnavigation without refueling. The Voyager completed a non-stop, unrefueled flight around the world in 1986, covering a distance of approximately 26,358 miles (42,432 kilometers).
Space Travel: If we consider the distance traveled by spacecraft as well, the Apollo missions to the Moon hold the record. The Apollo 13 mission, in particular, followed a trajectory that took it approximately 248,655 miles (400,171 kilometers) away from Earth as it looped around the Moon and returned to Earth.
These are just a few notable examples, and it's important to keep in mind that technological advancements and specific mission requirements can influence the distances covered by aircraft.