service in 1953 and the type served the US Army for almost forty years, the last U-8Fs being retired in 1992. When US military aircraft designations were revised in 1962, the remaining L-23D, RL-23D, L-23E and L-23F aircraft became U-8Ds, RU-8Ds, U-8Es and U-8Fs. A further 47 Model 65s were ordered in 1962 and 1963 as U-8Fs; one of these was delivered to the Pakistan Army, formally the only L-23 or U-8 delivery to a foreign user (although other nations bought Queen Airs off-the-shelf as military aircraft). These were the final new-build aircraft in the series, however the unique Beechcraft Model 87 turboprop proof-of-concept aircraft used in developing the King Air was delivered to the Army as the NU-8F in 1964 (this was later re-designated the YU-21) and a few used Queen Airs were also taken by the Army. Many U-8Fs were modified during their service lives to a similar standard as civilian Excalibur Queen Airs. The most obvious modifications are more powerful Lycoming IO-720 eight-cylinder engines in place of the factory-fitted six-cylinder engines; and bulged main landing gear doors that fully enclose the wheels when the gear is retracted instead of the wheels partially protruding through the doors. Many L-23Ds/U-8Ds and U-8Fs have been registered as civilian aircraft since retirement from military service. Variants YL-23 Four Model 50 Twin Bonanzas evaluated by the US Army subsequently converted to L-23A standard. L-23A 55 delivered to the US Army, many subsequently converted to L-23D standard. L-23B Based on B50. 40 delivered to the US Army from 1954, with many subsequently converted to L-23D standard. XL-23C One aircraft based C50 for evaluation by the US Air Force. L-23D 100 delivered new to the US Army plus 93 L-23As and Bs rebuilt to L-23D standard. RL-23D variant with belly-mounted radar. Eleven new-build aircraft delivered to the US Army, plus 29 converted from L-23Ds. L-23E Six delivered to the United States Army. L-23F 24 aircraft based on the Beechcraft Queen Air delivered to the US Army. U-8D L-23D redesignated in 1962. RU-8D RL-23D redesignated in 1962, plus 15 U-8Ds converted after 1962. U-8E L-23E redesignated in 1962. U-8F L-23F redesignated in 1962 and 46 new-build aircraft delivered to the US Army, plus single Beechcraft Model 87 delivered used as NU-8F in 1964 and one Queen Air purchased second-hand in 1966. In addition, one U-8F was delivered to the Pakistan Army instead of the US