We present the design, operating principle and performance of a room-temperature monolithic broadband terahertz (THz) source for applications of THz imaging and detection of  explosive materials such as TNT, RDX, PETN and HMX. The suggested terahertz source is a 20-element array of quantum cascade lasers (QCLs) emitting at discrete frequencies from 0.85 to 4.74 THz. The layer structure of each individual THz QCL is based on a two-well design scheme with variable barrier heights and resonant-phonon depopulation of the lower laser state. The tailoring of emission frequency of individual THz QCLs in the laser array was made by varying the constituent epilayers’ width and doping level of the injector well. We found that the peak optical gain of the discrete THz QCLs is increased with increasing tailored THz emission frequency.