We demonstrate a novel bandgap engineering approach in a new class of 2D layered materials, semiconducting transition metal dichalcogenides (TMDs). Our approach does not require sophisticated growth systems and the bandgap size of the channel material can be dynamically adjusted during the device operation to meet the requirement of a specific application. We will show that the transfer characteristics of a double-gated bilayer MoS2 field-effect transistor (FET) device can continuously change from semiconducting to metallic behavior with increasing gate field, indicating a dynamically reducing bandgap. In photoluminescence measurements, we also observe that the peak associated with the direct bandgap of MoS2 evolves into two resonances under electric field with one peak continuously shifting to lower energies, which is consistent with the interlayer transition induced bandgap reduction observed by transport.