There hasn't been a full secretary of defense since the resignation of James Mattis in December previous year after splits in the administration over Trump's sudden decision to remove USA troops from Syria.
Shanahan, a former Boeing executive with no prior experience in national security matters, has been leading the Pentagon as acting secretary since January 1, a highly unusual arrangement for arguably the most sensitive cabinet position.
Acting Defense Secretary Patrick Shanahan speaks at an event presented by the Center for Strategic and International Studies.
"The confirmation process should focus on securing our nation against threats, readiness and the future of our military, and ensuring the highest quality care and support for service members and their families", Shanahan said in a separate statement.
What we know about the Gulf tanker attacks
He suggested that those flying objects could have been bullets, and called reports of striking a mine " false ". Pompeo said: "Iran is lashing out because the regime wants our successful maximum pressure campaign lifted".
The president said, however, that they have a "very good vetting process" and this "came up a little bit over the last short period of time".
The president's tweet comes after several news outlets reported an incident of domestic violence in Shanahan's marriage, where he and his wife, who are now divorced, claimed they had been punched by the other, according to a USA Today story.
Trump told reporters Tuesday that Shanahan planned to take time off "for family matters", adding that he didn't ask him to withdraw his name from consideration.
Mr Shanahan, who will be replaced by Army Secretary Mark Esper, had not yet been nominated for the cabinet post.
Minutes after Mr Trump's announcement, the Washington Post published an interview with Mr Shanahan in which he discussed a 2011 incident when his son, then 17, reportedly beat his mother with a baseball bat fracturing her skull.
Joe Biden leads 2020 Democratic rivals in early states
Biden, meanwhile, ran for president twice - in 1988 and in 2008 - but lost to former Massachusetts Gov. Tim Ryan of Ohio, New York City Mayor Bill de Blasio, and Washington Gov.
Shanahan had a long career at Boeing's civil and military divisions before joining the Pentagon.
In Shanahan's tenure at the department he's had to deal with a wide array of global hotspots, ranging from missile launches by North Korea to the sudden shift of military ships and aircraft to the Middle East to deal with potential threats from Iran. Shanahan seemed to confirm that in a statement Tuesday afternoon, where he said that protecting his three children is his priority.
"His Pentagon experience includes serving as the Deputy Assistant Secretary of Defense (Negotiations Policy) in the Office of the Secretary of Defense and earlier on the Army staff as a war planner". The move appeared set to fill a void left after former Secretary of Defense Jim Mattis abruptly resigned late past year.
The United States has now been without a confirmed secretary of defense for more than six months, the longest in history.
Google Just Unveiled The Pixel 4 (And Its Camera Looks Amazing!)
Just yesterday, phone case models were shown off in a video that confirmed the square camera system on the rear. Officially confirming the Pixel 4, Google releases a sneak peek - more like a " leak " though.