The airborne differential absorption lidar LEANDRE II, developed for profiling tropospheric water-vapor mixing ratios, is described. The emitter is a flash-lamp-pumped alexandrite laser, which operates in a double-pulse, dual-wavelength mode in the 727–736 nm spectral domain. Two 50-mJ successive on-line and off-line pulses with an output linewidth of 2.4 × 10^-2 cm^-1 and a spectral purity larger than 99.99% are emitted at a 50-µs time interval. The spectral positioning is controlled in real time by a wavemeter with an absolute accuracy of 5 × 10^-3 cm^-1. The receiver is a 30-cm aperture telescope with a 3.5-mrad field of view and a 1-nm filter bandwidth. These instrument characteristics are defined for measuring the water-vapor mixing ratio with an accuracy better than 0.5 g kg^-1 in the first 5 km of the atmosphere with a range resolution of 300 m, integration on 100 shots, and an instrumental systematic error of less than 2%. The sensitivity study and first results are presented in part II [Appl. Opt.40, 3462–3475 (2001)].