KAGRA is a newly built gravitational wave observatory, a laser interferometer with 3~km arms, located in Kamioka, Gifu, Japan. KAGRA is constructed in the underground and is operated with cryogenic mirrors in order to reduce seismic and thermal noises, respectively. Both technologies are expected to show a future direction of gravitational-wave detectors. In 2019, KAGRA finished all the installations with designed configuration, which we call the baseline KAGRA. In this occasion, we overview the baseline KAGRA from various viewpoints as the series of articles. In this article, we introduce the design configurations of KAGRA, with its historical background.