This Plane Is Critical to the Air Force, But Not F-22 or F-35

The Air Force relies on a small but diverse fleet of tricked out Boeing 707 airliners with specialized sensors and communications links to perform vital reconnaissance and command-and-control missions across the globe.

Video Source: US Military Power

There are the last three types potential targets of despite having seen extensive use in the last 40 years. In part, it’s because of their size, age and large crews results in hefty operating costs. 

The U.S. Air Force still operates nearly 400 707-based KC-135 tankers and over 70 RC-135, E-3 and E-8 aircraft.