We present an overview of the Primordial Inflation Polarization Explorer (PIPER) – a balloon-borne telescope that measures the cosmic microwave background polarization on large angular scales in search of inflationary gravitational waves. PIPER consists of two co-aligned telescopes with 32⨉40 transition-edge sensors each mapping over half of the sky at four frequency bands between 200 to 600 GHz. A combination of fully cryogenic optics, rapid front-end polarization modulator, and background-limited detectors provides PIPER with high sensitivity and control over instrumental systematics necessary for detecting the primordial B-modes. Through two engineering flights in 2017 and 2019, we have validated the cryogenic environment at float altitude, operation of flight electronics, and recovery after landing. Starting at the end of 2021, a series of science flights have been planned from Ft Sumner, New Mexico.