We report a large-format (640 x 512) voltage-tunable quantum-well infrared photodetector (QWIP) focal plane array (FPA) for dual-color imaging in the midwavelength infrared (3-5 mu m) band. Voltage-tunable spectral response has been achieved through series connection of two eight-well stacks of AlGaAs-InGaAs epilayers grown by molecular beam epitaxy on GaAs substrate. The peak responsivity wavelength of the detectors is shifted from 4.1 mu m (color 1) to 4.7 mu m (color 2) as the bias is increased within the limit applicable by commercial read-out integrated circuits. The operability of the FPA is similar to 99.5% with noise equivalent temperature differences of similar to 60 and 30 mK (f/1.5) in color modes 1 and 2, respectively. To our knowledge, this is the first large-format voltage-tunable dual-color QWIP FPA reported for midwavelength thermal imaging.