Except for minor editorial revisions this paper is the reproduction of a report written for the Metallurgical Laboratory of the University of Chicago almost ten years ago, after the experimental production of a divergent chain reaction. This report has now been declassified and can be published.The present first part of the report contains a general description of the first pile and of its operation. The details of the construction, preparation, and testing of the materials and of the instrumentation are given by the members of the groups responsible for the work in Appendices I and II. The pile had approximately the shape of a flattened ellipsoid of graphite having 388-cm equatorial radius and 309-cm polar radius. The uranium was distributed through the graphite mass in lumps partly of metal and partly of oxide arranged in a cubic lattice array with about 21-cm cell side. The experimental procedure followed in approaching the critical dimensions and in the actual operation of the pile is described. The observed critical dimensions are compared with the expectation from the tests on the various components of the structure.