The desire for a large infrared space telescope traces back decades; in the United States the Shuttle Infrared Telescope Facility was planned while the Space Shuttle was in development and the potential for infrared astronomy was acknowledged at that time.
Compared to ground telescopes, space observatories were free from atmospheric absorption of infrared light; this would be a whole "new sky" for astronomers.

The tenuous atmosphere above the 400 km nominal flight altitude has no measurable absorption so that detectors operating at all wavelengths from 5 μm to 1000 μm can achieve high radiometric sensitivity.

— S. G. McCarthy and G. W. Autio, 1978
However, infrared telescopes have a disadvantage: they need to stay extremely cold and the longer the wavelength of infrared, the colder they need to be.
If not, the background heat of the device itself overwhelms the detectors, making it effectively blind.
This can be overcome by careful spacecraft design, in particular by placing the telescope in a dewar with an extremely cold substance, such as liquid helium.
This has meant most infrared telescopes have a lifespan limited by their coolant, as short as a few months, maybe a few years at most.
It has been possible to maintain a temperature low enough through the design of the spacecraft to enable near-infrared observations without a supply of coolant, such as the extended missions of Spitzer Space Telescope and Wide-field Infrared Survey Explorer.
Another example is Hubble's NICMOS instrument, which started out using a block of nitrogen ice that depleted after a couple of years, but was then converted to a cryocooler that worked continuously.
The James Webb Space Telescope is designed to cool itself without a dewar, using a combination of sunshield and radiators with the mid-infrared instrument using an additional cryocooler.

The telescope's delays and cost increases can be compared to the Hubble Space Telescope.
When Hubble formally started in 1972, it had an estimated development cost of US$300 million (or about US$1 billion in 2006 constant dollars), but by the time it was sent into orbit in 1990, the cost was about four times that.
In addition, new instruments and servicing missions increased the cost to at least US$9 billion by 2006.

In contrast to other proposed observatories, most of which have already been canceled or put on hold, including Terrestrial Planet Finder (2011), Space Interferometry Mission (2010), International X-ray Observatory (2011), MAXIM (Microarcsecond X-ray Imaging Mission), SAFIR (Single Aperture Far-Infrared Observatory), SUVO (Space Ultraviolet-Visible Observatory), and the SPECS (Submillimeter Probe of the Evolution of Cosmic Structure), the JWST is the last big NASA astrophysics mission of its generation to be built.
The desire for a large infrared space telescope traces back decades; in the United States the Shuttle Infrared Telescope Facility was planned while the Space Shuttle was in development and the potential for infrared astronomy was acknowledged at that time. Compared to ground telescopes, space observatories were free from atmospheric absorption of infrared light; this would be a whole "new sky" for astronomers.
The tenuous atmosphere above the 400 km nominal flight altitude has no measurable absorption so that detectors operating at all wavelengths from 5 μm to 1000 μm can achieve high radiometric sensitivity.
— S. G. McCarthy and G. W. Autio, 1978
However, infrared telescopes have a disadvantage: they need to stay extremely cold and the longer the wavelength of infrared, the colder they need to be. If not, the background heat of the device itself overwhelms the detectors, making it effectively blind. This can be overcome by careful spacecraft design, in particular by placing the telescope in a dewar with an extremely cold substance, such as liquid helium. This has meant most infrared telescopes have a lifespan limited by their coolant, as short as a few months, maybe a few years at most. It has been possible to maintain a temperature low enough through the design of the spacecraft to enable near-infrared observations without a supply of coolant, such as the extended missions of Spitzer Space Telescope and Wide-field Infrared Survey Explorer. Another example is Hubble's NICMOS instrument, which started out using a block of nitrogen ice that depleted after a couple of years, but was then converted to a cryocooler that worked continuously. The James Webb Space Telescope is designed to cool itself without a dewar, using a combination of sunshield and radiators with the mid-infrared instrument using an additional cryocooler.
The telescope's delays and cost increases can be compared to the Hubble Space Telescope. When Hubble formally started in 1972, it had an estimated development cost of US$300 million (or about US$1 billion in 2006 constant dollars), but by the time it was sent into orbit in 1990, the cost was about four times that. In addition, new instruments and servicing missions increased the cost to at least US$9 billion by 2006.
In contrast to other proposed observatories, most of which have already been canceled or put on hold, including Terrestrial Planet Finder (2011), Space Interferometry Mission (2010), International X-ray Observatory (2011), MAXIM (Microarcsecond X-ray Imaging Mission), SAFIR (Single Aperture Far-Infrared Observatory), SUVO (Space Ultraviolet-Visible Observatory), and the SPECS (Submillimeter Probe of the Evolution of Cosmic Structure), the JWST is the last big NASA astrophysics mission of its generation to be built.
James Webb Space Telescope