WASHINGTON — On March 16, the White House announced its intention to nominate Boeing executive Patrick Shanahan as deputy secretary of defense, the No. 2 spot in the Pentagon.

Two months later, the Senate is still waiting to receive his formal nomination, a notable delay for a Department of Defense that has only two Senate confirmable spots filled — and which has seen a pair of Army secretary nominees, as well as a Navy secretary nominee, drop out after being announced.

Shanahan's name was part of a group of six nominees announced on March 16, but the Boeing executive's delay is singular.

David Norquist, the nominee for comptroller, was officially nominated April 6; Norquist's deputy, Elaine McCusker, was nominated April 24; and Robert Daigle, to be director of the Cost Assessment and Program Evaluation office, was nominated April 25. The Senate Armed Services Committee held a hearing for all three on May 9.

Two others announced on the same day — David Trachtenberg for principal deputy under secretary of defense for policy and Kenneth Rapuano for assistant secretary of defense, homeland defense and global security — were officially nominated April 28 and April 25, respectively.

The delay for Shanahan looks even more out of place given that two more nominees — Robert Story Karem for assistant secretary for international security affairs and Kari Bingen as principal deputy under secretary for intelligence — were announced and sent to the Senate on April 25. Bingen, Karem and Rapuano will have their time in front of the SASC on Thursday.

A spokesman for Boeing directed questions about Shanahan’s status to the White House, which did not respond to requests for comment. However, sources say Shanahan has been in touch with various offices inside the Department of Defense and has had several informational briefings, including visits to the Pentagon, so it appears he remains the nominee-to-be.

What's behind the delay?

Senate Armed Services Committee Chairman John McCain, R-Ariz., said he did not know why Shanahan’s paperwork has not reached the committee but suggested there weren't concerns from members of the SASC, akin to those that scuttled the nomination of Tennessee state Sen. Mark Green to be secretary of the Army.

"He’s a Boeing executive, and I think that makes it more complicated, but we’d certainly like to move forward with him. He’s got an excellent reputation," McCain said.

A former DoD official who has helped out on several previous Pentagon transitions expressed a similar belief that unwinding all of Shanahan’s ties to Boeing may be slowing an already complicated process down. But it is also possible U.S. President Donald Trump's team, burned by a trio of failed nominees, may be expressing extra diligence before putting Shanahan’s name forward officially.

"It’s unfortunate that two months ago [Shanahan] was announced. They probably pulled the trigger a little too fast. Now they’ve been chastised, with the Mark Green and [Army secretary nominee Vincent Viola] situations," the former official said.

Arnold Punaro, a retired U.S. Marine general and former staff director on the Senate Armed Services Committee, also cautioned against seeing Shanahan’s delay as being particularly longer than other, process-driven delays in getting vetting done.

"Every administration would like their people sooner, and yes, we would like a more streamlined processing in both the executive and legislative branches, but that is not the reality of the complexities associated with the kinds of experienced, proven track record individuals we need in these very senior positions in government," Punaro said.

In the meantime, the role of deputy secretary of defense continues to be done by Bob Work, perhaps the highest-ranking holdover from former U.S. President Barack Obama's administration. Work is overseeing a double-digit number of reviews, part of a larger look at the Pentagon requested from the Trump administration.

If Shanahan takes several more weeks to receive a nomination hearing, it is possible some of those reviews will be completed before his arrival. And Punaro warns, "The Senate confirmation clock does not start until they actually get a nomination. And then for someone as senior as Shanahan, he will need  time to visit with Senators on the committee prior to his hearing."

Although McCain did not single Work out by name, he acknowledged that having an administration’s hand-picked official weigh in on an issue is different than having a holdover do so.

"We're not getting people to implement the new administration’s policies and strategies," McCain said. "That's the problem. You hire a team who is with your philosophically and to actively pursue the agenda. If someone is in an acting position, no matter how great their integrity, they just don't have the same kind of influence the way a regularly appointed member of the team does."

Joe Gould in Washington contributed to this report.