Blood leaked alpha - jboiler (full level walkthrough)
jboiler is a map in the leaked Blood alpha.
The most interesting thing about this map is that the titular boilers in this map appeared in the final game's E4M3: Charnel House as incinerators. I wouldn't go as far as to say this level evolved into E4M3, though, since jboiler's layout is otherwise very different from Charnel House's.
There are also hanging walkways about the level the player can use. Jumping off of these is required to reach the rear of the boilers and a path that is otherwise not accessible.
The j in the level's name is likely a reference to James Wilson III, who designed several maps in both the prototype and the final game. One of the maps in the prototype with "j" as it's first character has James Wilson III defined as the level's author, meaning this is probably the case.