The sixth-generation bomber is currently undertaking engine testing, described as an “essential milestone” on the path to an eventual first flight.

Unveiled in December last year, the B-21 Raider is the ‘sequel’ to the UFO-like B-2 Spirit and is designed to silently strike deep behind enemy lines with its 9,500 km range and advanced stealth capabilities.

Manufacturer Northrop Grumman has previously said the world has “never seen technology” like it has developed for the bomber, while US Defense Secretary Lloyd Austin has claimed it’s so advanced that even the most sophisticated air defence systems wouldn’t be able to detect it.

It comes after the Australian Federal Government surprisingly ruled out buying the B-21 Raider in its much-anticipated Defence Strategic Review.

Instead, the RAAF will invest in next-generation, long-range missiles that will be fired by Australia’s fleet of 72 F-35s and 24 Super Hornets.

“The review has undertaken detailed discussions in Australia and the United States in relation to the B-21 Raider as a potential capability option for Australia,” said the report.

“In light of our strategic circumstances and the approach to Defence strategy and capability development outlined in this review, we do not consider the B-21 to be a suitable option for consideration for acquisition.”

In place of its purchase, Australia will invest in Raytheon’s Joint Strike Missile (JSM) alongside Lockheed Martin’s Long Range Anti-Ship Missile (LRASM).

JSMs are designed to be fired from fifth-generation F-35s and are significantly able to change course in flight. They differ from more regular missiles because they can fly at low altitudes where they can evade radars.

Raytheon says the JSM, which has a range of 275km, is the only fifth-generation cruise missile designed to be launched from the internal weapons bay of the F-35A.

Lockheed’s LRASMs, meanwhile, with its range of 560km, use “semi-autonomous guidance and target cueing data” to locate and destroy targets. Unlike the shorter-range JSMs, they can be fired by both F-35s and Super Hornets.

The news that Australia will favour missiles over long-range aircraft came despite Defence Minister Richard Marles previously saying that purchasing the B-21 was “being examined” and US Air Force Secretary Frank Kendall suggesting his country would be “willing to talk” about a deal.

Think tank ASPI (The Australian Strategic Policy Institute) had estimated, though, that acquiring a fleet of 12 B-21s would cost Australia up to $28 billion.