SOFIA has reached in the last two years its full operational capabilities and is producing now great science on typically three observing flights per week. The telescope is the backbone of the observatory and is working nearly perfectly. This may be the right time to have a look on the design history of the telescope and some of the major subsystems, which ensure the functionality under the harsh aero-acoustic environment inside the aircraft cavity. A comparison with SOFIA’s predecessor KAO gives insight in to the challenges of airborne telescope design. The paper describes the development of the optical subsystem, the telescopes structure, the telescope mount and the interface to the aircraft from the conceptual design up to the finally as-built telescope, and comments on their influence on the overall observatory performance.