We describe a new low-frequency wideband radio survey of the southern sky. Observations covering 72-231MHz and Declinations south of +30{deg} have been performed with the Murchison Widefield Array "extended" Phase II configuration over 2018-2020 and will be processed to form data products including continuum and polarisation images and mosaics, multi-frequency catalogues, transient search data, and ionospheric measurements. From a pilot field described in this work, we publish an initial data release covering 1,447 sq. deg over 4h<RA<13h, -32.7deg<Dec<-20.7deg. We process twenty frequency bands sampling 72-231 MHz, with a resolution of 2'-45", and produce a wideband source-finding image across 170-231MHz with a root-mean-square noise of 1.27+/-10.15mJy/beam. Source-finding yields 78,967 components, of which 71320 are fitted spectrally. The catalogue has a completeness of 98% at ~50mJy, and a reliability of 98.2% at 5{sigma} rising to 99.7% at 7{sigma}. A catalogue is available from Vizier; images are made available on AAO Data Central, SkyView, and the PASA Datastore. This is the first in a series of data releases from the GLEAM-X survey.