  We present the XXL Survey, the largest XMM programme totaling some 6.9 Ms to
date and involving an international consortium of roughly 100 members. The XXL
Survey covers two extragalactic areas of 25 deg2 each at a point-source
sensitivity of ~ 5E-15 erg/sec/cm2 in the [0.5-2] keV band (completeness
limit). The survey's main goals are to provide constraints on the dark energy
equation of state from the space-time distribution of clusters of galaxies and
to serve as a pathfinder for future, wide-area X-ray missions. We review
science objectives, including cluster studies, AGN evolution, and large-scale
structure, that are being conducted with the support of approximately 30
follow-up programmes. We describe the 542 XMM observations along with the
associated multi-lambda and numerical simulation programmes. We give a detailed
account of the X-ray processing steps and describe innovative tools being
developed for the cosmological analysis. The paper provides a thorough
evaluation of the X-ray data, including quality controls, photon statistics,
exposure and background maps, and sky coverage. Source catalogue construction
and multi-lambda associations are briefly described. This material will be the
basis for the calculation of the cluster and AGN selection functions, critical
elements of the cosmological and science analyses. The XXL multi-lambda data
set will have a unique lasting legacy value for cosmological and extragalactic
studies and will serve as a calibration resource for future dark energy studies
with clusters and other X-ray selected sources. With the present article, we
release the XMM XXL photon and smoothed images along with the corresponding
exposure maps. The XMM XXL observation list (Table B.1) is available in
electronic form at the CDS. The present paper is the first in a series
reporting results of the XXL-XMM survey.
