SpaceX announced the Falcon Heavy on April 5, 2011. The basic concept was simple, strap two boosters on the side of a standard Falcon 9, and use it to increase the lift capacity. It also discussed a system to use a cross fuel system to fuel the center booster partially from the side cores, leaving to an even higher payload capacity. The estimated capacity in 2011 was between 20-55 tons to Low Earth Orbit. Originally, the first launch was planned for 2013. The original flight concept was as follows:
The first mission was constantly delayed for a number of reasons. The best theory I have heard is that the Falcon 9 continued to be developed in that period of time. There were 2 major revisions to the Falcon 9 design, as well as a number of smaller revisions. The theory goes that they didn’t want to re-design the Falcon Heavy for each new booster revision, and so they waited until it was close to completion prior to doing the Falcon Heavy Design.
The final version of Falcon Heavy has nixed a couple of the original ideas. First of all, there is no cross-feed version. Secondly, it will not be man rated, at least, that is the expectation. The successor to Falcon Heavy, the Big “Falcon” Rocket, is making good enough progress that they expect it will be used, and thus they don’t want to go through the expense to man rate the Falcon Heavy. Still it is able to carry more mass to Low Earth Orbit than any rocket in history, except for the Saturn V and Energia rockets, both of which are discontinued. It can carry more than twice as much as the nearest competitors. Even without cross feeding, it can still carry more mass to orbit than the original cross-fed version, at a much lower cost and risk. There are actually rockets that can lift more to very energetic orbits, such as to the Outer Solar System, but for Earth, Mars, the Moon, and Venus, it can’t be beat.
Falcon 9 can be used to prepare the way for future Human missions to anywhere in the Solar System. It could do an Apollo style mission, with two launches. It could carry at least 4 tons of cargo to Mars, and far more to the Moon, using the Dragon capsule. It developed this rocket for about $500 million, which is a small fraction of the cost of similar rockets that NASA has developed, or the military. The exact purpose for the rocket hasn’t yet been decided, but it is extremely capable. Right now mostly communication satellites are on the manifest, but no doubt more will be planned. The day that I am writing this, NASA just sent out a Request for Proposals to send cargo for a manned mission to the Moon. SpaceX no doubt will either pitch the Falcon Heavy, or else the BFR, to accomplish such a mission.