There may be variants developed for air freight, military or luxury corporate use. Occasionally the military version precedes the civilian airliner. Examples are the Boeing KC-135 and Boeing 707/720 series and, looking to the future, perhaps the Bell-Boeing MV-22 Osprey. Corporate or military use may follow the withdrawal of the airplane from use as a passenger transport, possibly for reasons of economy or environmental restraint. Most airliners are owned or leased by airlines.