The Middle-Jet is the Middleman's main mode of airborne transportation. It was stored at Middleman Headquarters, directly above the Middleboat.
Its most recent trip was to the Dread Pyramid of Itzilichlitlichlitzl where Wendy arrived to rescue Sensei Ping and the Middleman from the Mexican Luchadores. Wendy parachuted out of the Middlejet into the Dread Pyramid; it is unknown whether the Middlejet survived on its own or crashed into the jungles of Mexico. However, Wendy references the Middlejet in "The Obsolescent Cryogenic Meltdown", so at the least a Middlejet was back in service by then.
The Middlejet is a modified AV-8B Harrier II vertical takeoff and landing fighter jet.