Staff Writer (Updated: 4/5/2016):
The U-2 was designed to the highest degree of secrecy by famed Lockheed Aviator Kelly Johnson and his "Skunkworks" bureau. The initial system was airborne by 1955 with reconnaissance flights high above the Soviet Union beginning shortly thereafter. Now in the form of the U-2S model, the U-2 system can produce a large amount of photographic data with its highly capable intelligence-gathering suite in real-time. The aircraft traditionally flies above its 70,000 foot listed ceiling requiring the pilot to don a spacesuit-like outfit. The design is centered around the wide glider-like wingspan that is attached to the pencil-like fuselage. The cockpit offered up a less-than stellar field of vision, requiring the system to be guided to a landing by another airborne pilot trained in U-2 capabilities.
Since its inception, the U-2 has constantly been upgraded to improve performance and mission times. The system now sports a more friendly powerplant provided by General Electric that is efficient and lighter than previous systems. Avionics and other systems have been modernized to extend the life of the aircraft as well coming with the "Block 10" electrical systems upgrade. Additionally, the "Block 20" upgrade provided the U-2 system with a more modernized cockpit. More modifications are planned.
From 1992 onwards, all TR-1 and U-2 systems were designated under the single umbrella of "U-2". The United States Air Force maintains a contingent of 36 active U-2 systems including five two seat trainer types. Additionally, NASA operates two more U-2 units in the form of ER-2 for high-level research purposes. The U-2 system will most likely be retained for some more years (marked in some sources up to the year 2020) of service even though the initial U-2 was fielded in the 1950's.
In August of 2011, it was announced that the Global Hawk UAV will formally replace the aged U-2 systems in USAF service beginning in 2016, bringing an end to the tenure of the decades-old, long-winged "Dragon Lady". There have been some complaints by political forces indicating that it is still more cost-effective to field the U-2 over the Global Hawk and the U-2 still represents greater inherent capabilities over that of the UAV.
In January of 2015, it was announced that the U-2 will be retained for ongoing service in the high-altitude reconnaissance role and paired with an upgraded radar system. The decision to initially mothball the storied Cold War aircraft proved highly unpopular in many circles though it appears that the Global Hawk initiative for the USAF will still stay on pace thanks to improvements made to the system. As such, the U-2 fleet will function a bit while longer than originally anticipated.