The Learjet 25, first flown in prototype form on 12 August 1966, was basically a Learjet 24 incorporating a fuselage stretch of 1.27m to provide seating for the crew of two and eight passengers. Certificated on 10 October 1967, it was followed by improved Learjet 25B and Learjet 25C models in late 1970, the latter having additional fuel capacity; thus, the Learjet 25B and Learjet 25C corresponded respectively to the Learjet 24E and Learjet 24F. In 1976 incorporation of the new cambered wing and aerodynamic improvements introduced simultaneously on the Learjet 24 models resulted in new Learjet 25D and Learjet 25F versions of the basic Learjet 25 and, similarly, all four Learjets became available later with General Electric CJ610-8A turbojets certificated for operation at an altitude of 15545m.
In addition to civil operations, a number of Learjet 25s were acquired by air arms, including those of Argentina, Bolivia, Ecuador, Mexico, Peru and Yugoslavia. Their primary role in most cases was for high-altitude photography and remote sensing, with a large single or dual integral camera pod just forward of the wing. Such aircraft were convertible easily for high-speed cargo or VIP transport duties. Manufacture of the Learjet 25F ended during 1979, but the Learjet 25D continued to be built until August 1982, when the growing recession in the aircraft industry caused its production to be suspended indefinitely.
In January 1979 the company gained certification of the Learjet 28 and Learjet 29 Longhorn. Basically similar to the 10-seat Learjet 25D, these two models introduced a new wing of increased span. This incorporated a cambered leading edge and supercritical winglets to provide improved performance and cruise efficiency. The introduction of winglets displaced the wingtip fuel tanks of earlier models, which meant that all fuel had to be carried internally. Thus, the Learjet 28 had 10-seat accommodation, but the longer-range Learjet 29 traded two passengers for an additional 379 litres of fuel. However, wing manufacturing costs were considered prohibitive against the performance improvements gained and further development of both the Learjet 28 (five built) and 29 (two built) were terminated in August 1982. Around the same time, when sales of the Model 25 had reached 368, production was phased out in favour of the Learjet 35 and 36.
