NASA’s Wide Field Infrared Survey Telescope (WFIRST) is being designed to deliver unprecedented capability in dark energy and exoplanet science, and to host a technology demonstration coronagraph for exoplanet imaging and spectroscopy. The observatory design has matured since 2013 [“WFIRST 2.4m Mission Study”, D. Content, SPIE Proc Vol 8860, 2013] and we present a comprehensive description of the WFIRST observatory configuration as refined during formulation phase (AKA the phase-A study). The WFIRST observatory is based on an existing, repurposed 2.4m space telescope coupled with a 288 megapixel near-infrared (0.6 to 2 microns) HgCdTe focal plane array with multiple imaging and spectrographic modes. Together they deliver a 0.28 square degree field of view, which is approximately 100 times larger than the Hubble Space Telescope, and a sensitivity that enables rapid science surveys. In addition, the technology demonstration coronagraph will prove the feasibility of new techniques for exoplanet discovery, imaging, and spectral analysis. A composite truss structure meters both instruments to the telescope assembly, and the instruments and the spacecraft are on-orbit serviceable. We present the current design and summarize key Phase-A trade studies and configuration changes that improved interfaces, improved testability, and reduced technical risk. We provide an overview of our Integrated Modeling results, performed at an unprecedented level for a phase-A study, to illustrate performance margins with respect to static wavefront error, jitter, and thermal drift. Finally, we summarize the results of technology development and peer reviews, demonstrating our progress towards a low-risk flight development and a launch in the middle of the next decade.