The United States has taken a significant step closer to developing a nuclear bomb that some say could be the most dangerous ever produced.

The Department of Energy's National Nuclear Security Administration announced earlier this month that it has completed the development and testing phase of the B61-12, the first-ever guided nuclear bomb.

The news of the announcement was reported by Reveal.

The government now says that the so-called 'smart' nuclear bomb will enter the production engineering phase. By 2020, the US will commence full-scale production.

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Department of Energy says it has completed testing phase of B61-12 nuclear bomb. Sandia Labs mechanical engineer Ryan Schultz is seen above adjusting a microphone for an acoustic test

A front view of four nuclear free-fall bombs on a bomb cart. According to Reveal, the cost of the B61-12 program runs about $11billion for 400 units, making it the priciest nuclear bomb ever

'Reaching this next phase…is a major achievement for NNSA and the exceptionally talented scientists and engineers whose work underpins this vital national security mission', said NNSA Administrator Lt. Gen. Frank G. Klotz (Ret.).

'Currently, the B61 contains the oldest components in the US arsenal. This… will add at least an additional 20 years to the life of the system'.

The drive to complete preparations for the bomb's production is controversial given the misgivings of a number of former and current senior defense officials who say the precise nature of the weapon could provide a rationale for its future use in conflict.

According to Reveal, the cost of the B61-12 program runs about $11billion for 400 units, making it the priciest nuclear bomb ever.

Despite President Barack Obama's stated intention to scale down America's nuclear arsenal, the government has undertaken a massive 30-year, $1trillion project to modernize its atomic stockpiles.

'These life extension programs directly support President Obama's directive to maintain a safe, secure, and effective nuclear deterrent, while reducing the size of the stockpile', said Secretary of Energy Ernest Moniz.

'Once completed, the B61-12 LEP will allow for the retirement of the B83-1—the last megaton-class weapon in America's nuclear arsenal—while supporting the nation's continued commitment to our national security and that of our allies and partners'.

The cost of the B61-12 program runs about $11billion for 400 units, making it the priciest nuclear bomb ever. Training in dealing with the bomb is pictured above

The B61-12 differs from previous 'free-fall gravity' bombs in that it will be equipped with a tail kit manufactured by Boeing. The kit will enable a precision-guided trajectory.

It will also feature a mechanism that enables an adjustment of the explosive force – ranging from a high of 50,000 tons of TNT to a low of 300 tons.

The bombs can be delivered by stealth fighter jets.

'If I can drive down the yield, drive down, therefore, the likelihood of fallout, etc., does that make it more usable in the eyes of some – some president or national security decision-making process?', Gen. James Cartwright, the retired commander of the US Strategic Command, told PBS. 'And the answer is, it likely could be more usable'.

Last year, the air force tested a dummy version of the nuclear bomb, using a 'safe' version of the the B61-12 nuclear gravity bomb with no warhead

The B61-12 nuclear bomb was tested at Tonopah Test Range in Nevada. The controversial tests were designed to extend the lifespan of the weapon by upgrading some of its parts.

Tyler Keil, lead engineer for a test series using Sandia National Laboratories' Davis gun, performs a final diagnostics check on a data recorder for an impact test on the nose assembly of a mock B61-12 in a previous test. Data gathered from the test is helping analysts calibrate computer models.

The earlier tests were designed to test the forces needed for the warhead to detonate

Last year, the Air Force and the NNSA completed the first development flight test of the B61-12.

A 'safe' version of the the B61-12 nuclear gravity bomb with no warhead was tested at Tonopah Test Range in Nevada.

The tests were designed to extend the lifespan of the nuclear weapon by upgrading some of its parts.

This test was the first of three development flight tests for the B61-12 Life Extension Program (LEP).

'This test demonstrated successful performance in realistic flight environments followed by an effective release of a development test unit from a USAF F-15E from Nellis AFB. Telemetry, tracking and video data were successfully collected', the USAF said.

The test provides confidence in the weapon system and instrumentation system designs and the hardware at its current state prior to going to a baseline design review in 2016.

'This test marks a major milestone for the B61-12 Life Extension Program, demonstrating end-to-end system performance under representative delivery conditions', said NNSA Deputy Administrator for Defense Programs Dr. Don Cook.

'Achieving the first complete B61-12 flight test provides clear evidence of the nation's continued commitment to maintain the B61 and provides assurance to our allies'.

The B61, known before 1968 as the TX-61, was designed in 1963 by the Los Alamos National Laboratory in New Mexico.

THE HISTORY OF THE B61 BOMB The B61, known before 1968 as the TX-61, was designed in 1963 by the Los Alamos National Laboratory in New Mexico. Total production of all versions was approximately 3,155, of which approximately 1,925 remain in service as of 2002, and some 1,265 are considered to be operational The warhead has changed little over the years, although early versions have been upgraded to improve the safety features. It is believed there are 200 B61 bombs actively in use by the United States. Of these 200 bombs, 180 are deployed with NATO allies in Europe according to the 26 October 2013 issue of 'The Bulletin of the Atomic Scientists.' The B61-12 nuclear bomb undergoing earlier tests The newest variant is the B61 Mod 11, deployed in 1997, which is a ground-penetrating bunker buster. As of 2013, the Pentagon is asking for an $11 billion life-extension program for the B61 bomb, which would be the most ambitious and expensive nuclear warhead refurbishment in history. In January 2014, former Air Force Chief of Staff Norton Schwartz confirmed that the B61-12 nuclear bomb upgrade would have enhanced accuracy and a lower yield with less fallout compared to previous versions of the weapon. Advertisement

The B61-12 LEP entered Development Engineering in February 2012 after approval from the Nuclear Weapons Council, a joint Department of Defense and Department of Energy/NNSA organization established to facilitate cooperation and coordination between the two departments as they fulfill their complementary agency responsibilities for US nuclear weapons stockpile management.

According to details released at the time, it 'refurbishes both nuclear and non-nuclear components to extend the bomb's service life while improving its safety, security and reliability.' Maj. Gen. Sandra Finan, the commander of the Air Force Nuclear Weapons Center, said in an April press release, 'Our mission is still to deliver nuclear capabilities and winning solutions that warfighters use daily to deter our enemies and assure our allies'.