WASHINGTON — Northrop Grumman has rebuffed a request by Boeing to team up to develop America’s next intercontinental ballistic missile, according to the latter company.

The attempt comes months after Boeing dropped out of the running to compete directly with Northrop on the Ground Based Strategic Deterrent program, which is expected to cost about $85 billion over the life of the program.

“In our discussions to date, Northrop Grumman has expressed that they are not interested in partnering with Boeing to form a best-of-industry GBSD team,” Boeing spokesman Todd Blecher said in a statement. “We are increasingly concerned that the Air Force’s deterrence mission and the nation’s security will be deprived of the best solution — a proven approach that leverages both companies’ technical strengths and decades of ICBM experience."

In August 2017, Boeing and Northrop bested out Lockheed Martin to be the final two competitors on the program. But in July 2019, Boeing made the decision to drop out of the program, citing in part its belief that Northrop’s acquisition of solid-fueled rocket motor manufacturer Orbital ATK, now known as Northrop Grumman Innovation Systems, gave the competitor an unfair advantage.

In a July letter to the Air Force, the head of Boeing’s Defense, Space and Security division, Leanne Caret, said the current acquisition approach gives Northrop “inherently unfair cost, resource and integration advantages related to [solid rocket motors].”

“As I said in my July 8 letter, we lack confidence in the fairness of any procurement that does not correct this basic imbalance between competitors,” the CEO added.

Caret at the time said it was “not realistic” to expect Boeing and Northrop could develop a competitive, joint bid in the five months before proposals are due, given that both companies have been working on separate proposals for more than two years.

However, that tune seemed to change in recent weeks, with reports emerging that Boeing was hopeful to join onto the project after all. It also comes as external Air Force experts raised concerns that Northrop being the sole bidder on the GBSD could lead to increased costs or delays.

Northrop did not return Defense News’ request for comment by press time.