After several weeks of speculation that the two developers for the Air Force’s next-generation intercontinental ballistic missile system could work together on a path forward, the companies appear to be at a crossroads.

Northrop Grumman [NOC] has reportedly rejected Boeing’s [BA] offer to partner up to develop the Air Force’s Ground-Based Strategic Deterrent (GBSD) program, Boeing said in a Sept. 13 statement.

“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,” the statement said. “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.”

Boeing and Northrop Grumman were each awarded three-year development contracts in 2017 to fund GBSD prototypes, and both were expected to compete to produce the ground-based ICBM, with an engineering, manufacturing and development (EMD) contract scheduled for 2020.

Inside Defense first reported in July that Boeing confirmed it would not compete for the program, citing unfair advantages in the request for proposal. “While I appreciate the discussions we have had since that time, the final RFP issued on July 16 does not address Boeing’s core concerns with the competition,” said Boeing Defense, Space and Security CEO Leanne Caret in a July 23 letter to Air Force acquisition leaders. “After thorough evaluation, it is with great regret that I inform you that Boeing does not intend to submit a prime offer in the GBSD EMD procurement.”

Since the 2017 contracts were awarded, Northrop Grumman acquired solid rocket motor manufacturer Orbital ATK – now known as Northrop Grumman Innovation Systems – a move that Caret and analysts have said could provide the company with an advantage in the competition. The only U.S. manufacturers of solid rocket motors are NGIS and Aerojet Rocketdyne [AJRD]

Boeing has developed the Air Force’s Minuteman land-based ICBMs since the 1960s. The company has attempted to build a “best-of-industry” partnership with Northrop since its July 23 letter to move forward with the GBSD program. Northrop Grumman did not respond to a request for comment by Defense Daily’s deadline Friday. The Air Force did not provide a comment, but Will Roper, Air Force Assistant Secretary for Acquisition, Technology and Logistics refused to comment on the program Sept. 4 at the Defense News Conference, as it is technically currently in the source selection phase.