DARPA formulated the Explainable Artificial Intelligence (XAI) program
in 2015 with the goal to enable end users to better understand, trust,
and effectively manage artificially intelligent systems. In 2017, the
four-year XAI research program began. Now, as XAI comes to an end in
2021, it is time to reflect on what succeeded, what failed, and what was
learned. This article summarizes the goals, organization, and research
progress of the XAI Program.