The position originally was supposed to go to a civilian

That's an interesting idea but if that were true then why was the first Secretary of Defense James Forrestal who was a Lieutenant in the Navy?

Edit: So I did some more research on this and it seems like most Secretary of Defense were not generals. I went off Wikipedia so bare that in mind, break down it below.

MIL Rank

Lieutenant x3

General x3

Lieutenant Colonel x2

First Lieutenant x3

Captain x4

Sergeant x1

Colonel x1

Unknown x1 (Bill Clements)

CIV

Patrick M. Shanahan (Business, Politician)

James R. Schlesinger (Economist)

Harold Brown (Physicist)

Dick Cheney (Politician)

William Cohen (Business, Politician)

Ashton Carter (Physicist, Politician)

David Norquist (Politician)

The position originally was supposed to go to a civilian

That's an interesting idea but if that were true then why was the first Secretary of Defense James Forrestal who was a Lieutenant in the Navy?