\documentclass{standalone}
\usepackage{tikz}
\usepackage{siltex}
\usetikzlibrary {arrows.meta,positioning,calc,math}
\tikzset{nodes={text height=.7em, text width=2.7cm, align=center,
draw=black!50, thick, font=\footnotesize, fill=white},
>={Stealth[round,sep]}, rounded corners, semithick}

\pgfdeclarelayer{bg}
\pgfsetlayers{bg,main}

\begin{document}
\begin{tikzpicture}[on grid,y=1.2cm,x=2.9cm]
\def\rectoffset{-1.2mm,2.5mm};
\def\rectoffsetleft{-2.90cm,0cm};
\def\textoffset{2.9cm,0cm};
\def\phasegroup(#1,#2,#3){%
    \begin{scope}[nodes={draw=none, fill=none,text height={}}]
        \coordinate (a) at ($(#1.north west)+(\rectoffset)$){};
        \coordinate (b) at ($(#2.south east)-(\rectoffset)-(\rectoffsetleft)$){};
        \draw[fill=lightgray] (a) rectangle (b);
        \node[anchor=center] at ($(#1)+(\textoffset)$) {#3};
    \end{scope}
    };
    \node (pd) {Problem Description};
    \node (sp)[below=1 of pd]  {Requirements};
    \node (id)[below=1 of sp]  {Initial Design};
    \node (fs)[below=1 of id]  {Feature Definition};
    \node (tp)[below=1 of fs]  {Test Protocol};
    \node (ss)[below=1 of tp]  {Feature Selection};
    \node (a1)[below=0.8 of ss,draw=none, fill=none] {};
    \node (rd)[below=0.8 of a1]  {Rapid Development};
    \node (va)[right=1 of a1]  {Variable Approach};
    \path[->] (pd) edge (sp)
              (sp) edge (id)
              (id) edge (fs)
              (fs) edge (tp)
              (tp) edge (ss)
              (ss) edge (rd)
              (rd.east) edge[bend right] (va)
              (va) edge[bend right] (ss.east);
    \begin{pgfonlayer}{bg}
        \phasegroup(pd,id,Systems Engineering);
        \phasegroup(fs,rd,Rapid Iterative Design Method);
    \end{pgfonlayer}
\end{tikzpicture}
\end{document}