The HALO project is based on an initiative of the entire German atmospheric science community. It was found that the acquisition of a modern research aircraft is essential to continue the successful research and maintain the leading position of German scientists for the next decades in this area. DLR (Deutsches Zentrum für Luft- und Raumfahrt) and MPG (Max-Planck-Gesellschaft) took the lead and submitted the application for this major research facility investment. This project is supported by scientists from 31 institutes. The HALO proposal was evaluated by the scientific advisory body of the Federal Government (Wissenschaftsrat) who recommended the realisation of HALO without restrictions. In September 2004 the German ministry of education and research has approved the construction and modification of this high-performance research aircraft. The costs of the HALO aircraft will be covered by contributions from the Ministry of Education and Research, the Helmholtz Association and the Max-Planck Society. HALO will be based on a GulfstreamG550 - a large business jet. However before the aircraft will take off for the study of scientific questions it will undergo significant modifications that transform the luxury passenger aircraft into a research platform.