X-band Resonant Reflector (RR) based Backward Wave Oscillators (BWO) has been designed and tested on a Marx generator. Output power dependence on the Resonant Reflector-BWO separation as well as on the applied magnetic field has been investigated using particle-in-cell (PIC) simulation. It was observed that the output power gets maximized for 3.0 T guiding magnetic field. The PIC simulation predicted BWO frequency as 9.22 GHz. BWO is operated on a Marx generator with peak diode voltage of 400 kV, and 4.0 kA as peak current. Microwave power was measured by D-dot probe placed in the far field region of the transmitting antenna. The measured peak output power was 120MW at 9.35 GHz frequency for 0.7 T guiding magnetic field. At high magnetic field of 3.0 T the BWO peak power increased to 350MW, however the frequency reduces to 9.25 GHz.