This paper proposes the Flying Test Bed (FTB) for the development of the ATREX engine. The ATREX engine is the air breathing engine, which is under development at Japan Aerospace Exploration Agency (JAXA) for its possible use as a main engine of the next generation space transportation system. Since the engine is the air breathing, some of development tests should be conducted under the real flight environment using the Flying Test Beds (FTBs). The authors studied the concept of the FTB and propose the solid motor assisted FTB system. Since the essential development data can be observed at hypersonic regions, it must be reasonable to accelerate the ATREX engine to Mach 2 or more with the conventional propulsion system such as solid booster. The authors have studied the possibility of solid motor based FTB and concluded that it is ideal for the development of ATREX engines.