\documentclass{jarticle}
\usepackage{amsmath}
\usepackage{accents}
\usepackage{statex}

\makeatletter
\def\widebar{\accentset{{\cc@style\underline{\mskip10mu}}}}
\def\Widebar{\accentset{{\cc@style\underline{\mskip8mu}}}}
\makeatother


\begin{document}

$
\bar{X}_i
\overline{X}_i
\widebar{X}_i
\Widebar{X}_i
\b{X_i}
\ol{X_i}
$


\begin{align*}
& \bar{X}_i        \\
& \overline{X}_i   \\
& \widebar{X}_i    \\
& \Widebar{X}_i    \\
& \b{X_i}          \\
& \ol{X_i}
\end{align*}


\end{document}
