Correct me if I am wrong, this is the first time in the MCU we were told exactly how "this Hulk" (technically Norton's Hulk) came to be. IIRC from the Ang Lee movie the experiment wasn't about trying to duplicate the super-soldier process but something else entirely. In fact the opening credits sequence to TIH suggests Banner willingly tested the process on himself. (In the Ang Lee movie, Banner is exposed to the Gamma Radiation accidentally when tries to save a co-worker from exposure apparently thinking his body is made out of lead.)
And, yeah, I realize the "Norton Hulk Origin" footage is available as a DVD extra or something, but I've never seen it.
Let's go by parts:
- In the Ang Lee movie, which is not connected in any way to the MCU, Banner (Kessler, sigh) was investigating, along with Betty Ross, using nanobots to heal injuries. It was a civilian investigation in an university, no military involved until the movie starts and former major Talbot tries to acquire their research.
The gamma radiation from the accident combined with the nanobots and the genetic modifications transmited to Bruce by his father create the Hulk.
- In the Inc. Hulk, Banner and Betty were working at Culver University, under the sponsorship of the US Military in a project to "strengthen cellular resistance to radiation" in soldiers (or so they are told).
General Ross actually was using Banner & Betty's research as part of restarting the Bio Force Enhancement Project (the Super-Soldier Program).
Banner, under pressure of funding being cut, tests the process on himself, iirc.
All of that is explained in two scenes, the one in which General Ross & Blonsky discuss what Banner is, and an scene between Betty and Banner, in which he tells her that her father wanted to weaponize the Hulk.
Here we go:
Gen. Thaddeus 'Thunderbolt' Ross: You must realize that what I'm about to tell you is very sensitive, both to me and to the Army. You know that we have a Bio-Force Department, and that we had a bio-force enhancement research project developed during World War II...
Emil Blonsky: A Super-Soldier.
Gen. Thaddeus "Thunderbolt" Ross: A simplification, but yes. We decided to dust it off, and give it another go, aiming to create the better soldier. Banner’s work was very early phase. It wasn’t even weapons application. He thought he was working on radiation resistance. I would never have told him what the project really was. But he was so sure of what he was onto, that he tested it on himself. And something went very wrong. Or it went very right. As far as I’m concerned, that man’s whole body is property of the US Army.
Emil Blonsky: You said he wasn’t working on weapons, right?
Gen. Thaddeus "Thunderbolt" Ross: No.
Emil Blonsky: But you were. You were, weren’t you? You were trying other things.
Gen. Thaddeus "Thunderbolt" Ross: One serum we developed… was very promising.
Emil Blonsky: So why did he run?
Gen. Thaddeus "Thunderbolt" Ross: He’s a scientist. He is not one of us.