\documentclass[landscape]{article}

\usepackage{longtable}

\begin{document}
\pagestyle{empty}

%{\bf\hfill B-FIELD SIGNS FOLLOWING ARITHMETIC OPERATIONS\hfil}

\setlength{\LTcapwidth}{5in} % LongTable caption width, default was 4in
\renewcommand{\arraystretch}{2}

\begin{longtable}[l]{l|c|c|c|c|c|c|c|c|c|}
\caption{\bf B-FIELD SIGNS FOLLOWING ARITHMETIC OPERATIONS}
\endhead
\cline{2-10}
 & \multicolumn{3}{c|}{Unsigned} & \multicolumn{3}{c|}{+ Sign} & \multicolumn{3}{c|}{$-$ Sign} \\
 & \multicolumn{3}{c|}{B field}  & \multicolumn{3}{c|}{B field} & \multicolumn{3}{c|}{B field} \\
\cline{2-10}
\cline{2-10}
 & \multicolumn{3}{c|}{if value is} & \multicolumn{3}{c|}{if value is} & \multicolumn{3}{c|}{if value is} \\
\cline{2-10}
                                     & A $<$ B & A = B & A $>$ B & A $<$ B & A = B & A $>$ B & A $<$ B & A = B & A $>$ \\
\hline
\multicolumn{1}{|l|}{ADD UNSIGNED A} & NS      & NS    & NS      & +       & +     & +       & $-$     & $-0$  & +     \\
\hline
\multicolumn{1}{|l|}{ADD +A        } & NS      & NS    & NS      & +       & +     & +       & $-$     & $-0$  & +     \\
\hline
\multicolumn{1}{|l|}{ADD $-$A      } & +       & +0    & $-$     & +       & +0    & $-$     & $-$     & $-$   & $-$   \\
\hline
\multicolumn{1}{|l|}{SUB UNSIGNED A} & +       & +0    & $-$     & +       & +0    & $-$     & $-$     & $-$   & $-$   \\
\hline
\multicolumn{1}{|l|}{SUB +A        } & +       & +0    & $-$     & +       & +0    & $-$     & $-$     & $-$   & $-$   \\
\hline
\multicolumn{1}{|l|}{SUB $-$A      } & NS      & NS    & NS      & +       & +     & +       & $-$     & $-0$  & +     \\
\hline
\end{longtable}

\end{document}
