The AlInAs/InGaAs material system is promising for mid-wavelength infrared (MWIR) and multi-band quantum well infrared photodetectors (QWIPs) as a lattice-matched alternative to the strained AlGaAs/InGaAs system. In this paper, we report a large format (640 x 512) AlInAs/InGaAs QWIP focal plane array (FPA) with 4.9 mu m cut-off wavelength and assess the performance of this material system for MWIR QWIP applications at both pixel and large format FPA level. We also experimentally demonstrate that the cut-off wavelength of AlInAs/InGaAs QWIPs can be tuned in a sufficiently large range in the MWIR atmospheric window by changing only the quantum well (QW) width at the lattice-matched composition. The cut-off wavelength is shifted from 4.15 to 4.9 mu m when the QW width is decreased from 30 to 23 angstrom, in which case a very broad spectral response (Delta lambda/lambda(p) = 32%) with a reasonably high peak detectivity (5.5 x 10(10) cm Hz(1/2) W-1, f/1.5) is achievable. The AlInAs/InGaAs QWIP FPA yielded excellent sensitivity with a noise equivalent temperature difference as low as 22 mK and a background limited performance (BLIP) temperature as high as 115 K with f/1.5 aperture and 300 K background. The results clearly demonstrate the potential of this material system for completely lattice-matched dual-or multi-band QWIP FPAs for third-generation thermal imagers.