The company is offering 90 million in common stock and $5 billion in mandatory convertible securities.
Based on Friday's closing price, Boeing can raise $13.95 billion from the common stock offering, though such issues are typically priced at a discount to ensure enough demand.
The company's shares were last down 2% in premarket trading on Monday.
"The offering is certainly favorable for credit quality. We'll factor it into our assessment of the rating in the context of continued negative free cashflow," Ben Tsocanos, aerospace director at S&P Global Ratings, said.
A capital raise is essentially for Boeing to preserve its investment-grade credit rating. Rating agencies have warned that a prolonged strike may lead to a downgrade in the planemaker's credit rating, likely pushing up the cost of capital.
The move will boost Boeing's battered finances, which have worsened since roughly 33,000 of its workers represented by the machinists union walked off their jobs in September, halting production of models including its cash-cow 737 MAX aircraft.
The planemaker was already reeling under a regulator-imposed cap on production of its MAX jets after a January mid-air panel blowout.
The combination of labor woes and its production problems have caused it to burn cash in the last three quarters. Last week, the company reported a $6 billion third-quarter loss and said it would burn cash next year.
The same day, striking workers rebuffed an improved contract as it fell short of their demands of a 40% wage hike and restoration of a defined-benefit pension plan, which Boeing is unlikely to reinstate.
Earlier this month, Boeing entered into a $10 billion credit agreement with banks and announced plans to raise up to $25 billion through stock and debt offerings.
S&P Global has warned of a ratings downgrade if Boeing slipped below target cash balance of $10 billion or if the company had to increase leverage to meet debt maturities.
Boeing, which has never fallen below the investment-grade rating, had cash and marketable securities of $10.50 billion as of Sept. 30.
It has $11.5 billion of debt maturing through Feb. 1, 2026, and is committed to issuing $4.7 billion of its shares to acquire Spirit AeroSystems (SPR.N) and assume its debt.
Reuters had reported earlier this month Boeing was examining options to raise billions of dollars through a sale of stock and equity-like securities.
Boeing said on Monday it intends to use proceeds for general corporate purposes, which may include paying off debt.
