  Studies of gas-phase radial metallicity profiles in spirals published in the
last decade have diminished the importance of galactic bars as agents that mix
and flatten the profiles, contradicting results obtained in the 1990s. We have
collected a large sample of 2831 published HII region emission-line fluxes in
51 nearby galaxies, including objects both with and without the presence of a
bar, with the aim of revisiting the issue of whether bars affect the radial
metal distribution in spirals. In this first paper of a series of two, we
present the galaxy and the HII region samples. The methodology is homogeneous
for the whole data sample and includes the derivation of HII region chemical
abundances, structural parameters of bars and discs, galactocentric distances,
and radial abundance profiles. We have obtained O/H and N/O abundance ratios
from the Te-based (direct) method for a sub-sample of 610 regions, and from a
variety of strong-line methods for the whole HII region sample. The strong-line
methods have been evaluated in relation to the Te-based one from both a
comparison of the derived O/H and N/O abundances for individual HII regions,
and a comparison of the abundance gradients derived from both methodologies.
The median value and the standard deviation of the gradient distributions
depend on the abundance method, and those based on the O3N2 indicator tend to
flatten the steepest profiles, reducing the range of observed gradients. A
detailed analysis and discussion of the derived O/H and N/O radial abundance
gradients and y-intercepts for barred and unbarred galaxies is presented in the
companion Paper II. The whole HII region catalogue including emission-line
fluxes, positions and derived abundances is made publicly available on the CDS
VizieR facility, together with the radial abundance gradients for all galaxies.
