Planes were vehicles that flew through the air. However, in the Destroy All Humans! series, they were only seen on land.
The first appearance of Planes. But only one was found at the Area 42 Airfield. It appeared to be a Cargo Plane or a Bomber that was based on the real life B29 Superfortress. It was possible that it got destroyed as well as the rest of the Airfield by the nuclear explosion that Crypto caused with the Nuke Truck.
During the mission, "Requiem for a Ratpoo", Orthopox ordered Crypto to destroy everything on Fantasy Atoll that had something to with Mr. Pork, including a plane. The Plane appeared to be a civilian version of a Martin PBM Mariner.
Technically speaking, the Planes did not appear in that game, but were instead mentioned during the mission, "Crypto Presents Disaster" where News Reporter Veronica Stone reported that the destruction of Sunnywood was caused by Soviet Fighter Jets and that World War III had begun.