Mullenberg was an aerospace engineer [1]. Two before him was Stonecipher, a physicist [2], who took over from Condit, another engineer [3].

[1] https://en.m.wikipedia.org/wiki/Dennis_Muilenburg#Early_life...

[2] https://en.m.wikipedia.org/wiki/Harry_Stonecipher#Early_life...

[3] https://en.m.wikipedia.org/wiki/Philip_M._Condit#Education