The GRAND project aims to detect ultra-high-energy neutrinos, cosmic rays and gamma rays, with an array of $200,000$ radio antennas over $200,000\,{\rm km}^2$, split into $\sim 20$ sub-arrays of $\sim 10,000\,{\rm km}^2$ deployed worldwide. The strategy of GRAND is to detect air showers above $10^{17}$\,eV that are induced by the interaction of ultra-high-energy particles in the atmosphere or in the Earth crust, through its associated coherent radio-emission in the $50-200$\,MHz range. In its final configuration, GRAND plans to reach a neutrino-sensitivity of $\sim 10^{-10}\,{\rm GeV}\,{\rm cm}^{-2}\,{\rm s}^{-1}\,{\rm sr}^{-1}$ above $5\times 10^{17}$\,eV combined with a sub-degree angular resolution. GRANDProto300, the 300-antenna pathfinder array, is planned to start data-taking in 2021. It aims at demonstrating autonomous radio detection of inclined air-showers, and study cosmic rays around the transition between Galactic and extra-Galactic sources. We present preliminary designs and simulation results, plans for the ongoing, staged approach to construction, and the rich research program made possible by the proposed sensitivity and angular resolution.