"So, I’m hopeful that we can fly both of those [the Tactical Boost Glide and Hypersonic Air-breathing Weapon Concept hypersonic missile projects] by the end of ’19 [but] it may slip into the early ’20 time-frame", Walker said on Wednesday at a news conference in Washington.
Last year, Michael Griffin, the US Pentagon's undersecretary of defence for research and engineering, said that US hypersonic capabilities were the highest technical priority for the country.
READ MORE: US Planning Five Hypersonic Test Programs in Marshall Islands
Hypersonic missiles, which fly at least five times faster than the speed of sound (1,234 km/hour), travel at high altitudes and may be launched from land, sea or air. Along with the United States, Russia and China are also currently leading the way in developing hypersonic capabilities.