The 135 isn't the only "old timer" in the Air Force fleet. The B-52 is projected to have a service life of nearly 90 years (first entered active service in 1955, expected to be flying until 2040). From Wikipedia:

Future of the B-52

Even while the Air Force works on new bombers scheduled for 2018 and 2037 it intends to keep the B-52H in service until at least 2040, nearly 80 years after production ended. This is an unprecedented length of service for a military aircraft.[4][74] B-52s are periodically refurbished at the USAF maintenance depots such as Tinker Air Force Base, Oklahoma.

The USAF continues to rely on the B-52 because it remains an effective and economical heavy bomber, particularly in the type of missions that have been conducted since the end of the Cold War, mainly against nations that have limited air defense capabilities. The B-52's capacity to "loiter" for extended periods over (or even well outside) the battlefield, while delivering precision standoff and direct fire munitions, has been a valuable asset in conflicts such as Operation Enduring Freedom in Afghanistan.

The speed and stealth of the B-1 Lancer and B-2 Spirit have only been useful until enemy air defenses were destroyed, a task that has been swiftly achieved in recent conflicts. The B-52 boasts the highest mission capable rate of the three types of heavy bombers operated by the USAF. Whereas the B-1 averages a 53% ready rate, and the B-2 achieved a 26%, the B-52 averages 80% as of 2001.[67]



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