The Birth: Way back when Marvel Comics just came out, there was no Spider-Man, no Hulk or nobody else. The Fantastic Four was the begining of Marvel Comics and was created by Stan "The Man" Lee and Jack "King" Kirby. Stan was the writer and Jack was the artist.
And with this new comic; Fantastic Four #1 (vol 1.) Dr.Reed Richards made his debut. Reed like the other members was just a mere human. He was also one of the top scientists in the US. It all started when an angry Ben Grimm was goaded by Susan Storm into helping Dr. Reed Richards pilot his spaceship. That night, four civilians; Reed Richards, Benjamin Grimm, Johnny Storm and Susan Storm took off in the spaceship Reed Richards had spent years constructing.
Benjamin Grimm may have been an excellent pilot and was steering the ship real smoothly, but fate had other plans for them. Their ship entered the cosmic storm area. Unfortunately for the forsome, their ship needed heavier shielding. The cosmic rays bombarded the ship running right through the ship's passengers, putting all of them into a tizzy. Ben felt too heavy to move and lost his control over the ship. Luckily the ship's autopilot kicked in and steered it to a rough but non-fatal landing.
It was only after the four exited the ship that they realized that the cosmic rays had affected them in a really drastic manner. It was Sue who first displayed signs of the after effect of the cosmic bombardment. She started fading away and then all of a sudden returned to normal. The next to experience the change was Ben. His skin started hardening and turned to an orange color. And then Reed started showing an elastic capability. The last to change was Johnny who started burning up and changed into a human torch
As realization of what had happened started to dawn on them, the forsome decided to use their powers for good. Dr. Reed Richards made himself the team leader and decided to call the group as The Fantastic Four and kept the name Mr. Fantastic for himself.

The Alter Ego: Like other members of the Fantastic Four, Reed Richards never made his indentity as Mr.Fantastic a secret to the public. When the team was first formed, Reed was already engaged to Susan Storm. Later the two got married and bore a son; Franklin. Reed's marriage with Susan has been quite stable with the normal after-marriage tussles.
Reed Richards happens to be one the the world's most brilliant and creative scientists. Most of the Fantastic Four's equipment has been designed by him. Infact most of his time is spent on scientific research. Because of his intellect and genius, he was readily accepted as the team leader.

The Legend: As Mr.Fantastic, Reed has the ability to stretch his body. There is almost no shape into which he cannot mould his plastic-like body. Because of his constant training he can make these changes at the speed of thought. His flexibility allows him to absorb any sort of impact. He can even catch bullets or turn them aside. He can use his body as a sort of a catapault for both objects and people; this catapaulting ability can be used to fire projectiles back at their original firer. He often uses his body to wrap around people that he wishes to detain. He can reduce his molecular cohesion, and become a "blob" of sorts. He can expand his body and use it as a makeshift glider. He can slim down enough for his body to fit through the eye of a needle. He cannot stretch without limit though. He can strerch for just over 100 yards before the onset of pain.
Reed's costume stretches with him. It is made of a special fabric invented by Reed which is made of unstable molucules. These molecules shift so that the fabric follows Mr.Fantastic's motions.

Reed Richards, ever since becoming the leader of the Fantastic Four, has directed most of his efforts towards the protection of Earth and his scientific experiments.
Reed set up the team's headquaters at the Baxter Building. After the Baxter Building was blown up by Kristoff (a ward of Doctor Doom with a nasty habit of thinking he is Doom) in Fantastic Four #278. It was replaced with Four Freedoms Plaza, but that too was irreparably damaged in THUNDERBOLTS #10. The FF have since relocated to a new HQ, Pier Four.
Reed, with Johnny's help, built the Fantasticar, the team's most famous mode of transport. They also use the pogo plane, a ship with vertical take-off and landing.

He has encountered several super villains; most notably his arch nemesis; Dr.Doom. Doom is a man of his word, which Reed Richards has found useful on many occassions. Another being they often encounter is Galactus, a huge being from another planet that actually feeds on the energy of planets.
At one time, when the Fantastic Four were in Latveria fighting an incredibly dagerous monster, Dr. Doom was about to get killed. In a heroic move to save Doom, Mr.Fantastic rushed to save him. Unfortunately both, Doom and Reed were killed in an explosion, though no bodies were found. The now Fantastic Three had a tough time dealing with Reed's death and also in deciding who will be their new leader. Much later, in issue #407, Dr. Reed Richards returns home. Seems he somehow had been transported to some primitive time and had been trying to find his way home. The main person responsible for this was a new and deadly villain known as Hyperstorm.
After the Fantastic Four battle it out with Hyperstorm, Reed then is challeged by Namor; the Sub-Mariner for the love of Susan Richards. Reed and Namor duked it out without bothering to find out what Sue thought. Of course, ultimately, the two original lovebirds, Reed and Sue come back together.
Though it's been some time since they battled Hyperstorm, this threat was not completely eliminated. Nathaniel Richards, Reed's father wanred Reed that in all the timelines that he observed, the Fantastic Four vanish completely from reality, most presumable destroyed by Hyperstorm. Reed then decides to tackle this problem by bringing a famished Galactus in front of Hyperstorm. Galactus tries to consume Hyperstorm, but Hyperstorm turns out to have an unlimited power source from the hyperspace from which he taps into. Reed then closes the portal as a result of which, both Hyperstorm and Galactus are exiled in a parallel dimension where Galactus will consume Hyperstorm indefinately.

About this time, Franklin Richards; Reed's son, makes friends with "Charlie"; another kid. But no-one else could see this Charlie, and thus the Fantastic Four thought that he was just Franklin's imaginary friend. What no-one knew was that Charlie was infact, Onslaught in disguise.
This is where the Onslaught saga starts. Onslaught finally kidnaps young Franklin. This is when the Fantastic Four realize that Onslaught is a real deadly threat; even though they were forwarned by the Avenger and X-Men. Mr. Fantastic bulit a machine that would shatter Onslaught's psionic control over the city. His device worked but eventually, the heroes get defeated and the Fantastic Four get transported to an alternative dimension - The Heroes Reborn Universe.
Some time back, the team with all other heroes returned from the alternate universe in which they were trapped.
More recently, Dr.Doom returned with his band of villains and attacked the Four. In the end, Mr.Fantastic faced Doom but somehow the two were carried away due to a time rupture. A few minutes later only Doom walked out of the explosion. The fact was that it was Mr.Fantastic who walked out trapped in Dr.Doom's armour.
Mr.Fantastic kept up the charade in fromt of Doom's super-generals while Susan and the rest of the team knew about the deception. But then changes seem to overtake Reed Richards and he started to think like Dr.Doom. Currently, Reed has been possessed by the armor and goes on referring to himself as Doom.

The Fantastic Four #1