We present the development of a new airborne mass spectrometer AIMS-H₂O for the fast and accurate measurement of water vapor in the upper troposphere and lower stratosphere. The high accuracy needed for e.g. quantification of atmospheric water vapor transport processes or cloud formation is achieved by an in-flight calibration of the instrument. AIMS-H₂O is deployed on the DLR research aircraft HALO and Falcon where it covers a range of water vapor mixing ratios from 1 to 500 ppmv.