By Shankar Ramakrishnan, Echo Wang and Tim Hepher

(Reuters) -Boeing is closing in on a plan to raise around $15 billion with common shares and a mandatory convertible bond as the jetmaker bolsters finances worsened by a crippling strike, but the timing remains unclear, four sources familiar with the matter told Reuters.

The company said on Tuesday in regulatory filings that it could raise as much as $25 billion in stock and debt with its investment-grade credit rating at risk. One of the sources cautioned that a $15 billion sale may not be enough for Boeing to address its ongoing crises.

Boeing is also considering a structured finance transaction to raise up to $5 billion that could resemble the securitization of a portion of a subsidiary's revenue, according to a separate source familiar with its financing plans. Boeing did not respond immediately to a request for comment on the securitization plan, which had not been reported previously.

The aerospace giant has been dealing with increased regulatory scrutiny, production curbs and a loss of confidence from customers since a door panel blew off a 737 MAX plane in midair in early January. Shares gained 1% on Wednesday, but are down more than 40% this year.

Boeing has been burning through cash all year, leading to its Tuesday announcements that it will raise money in the capital markets and that it had also secured a $10 billion credit agreement with major lenders: Bank of America, Citibank, Goldman Sachs and JPMorgan.

Four investor and banking sources said representatives from those lenders were inquiring about appetite for a combined offering of new shares and a mandatory convertible bond – a hybrid bond that could convert into equity on or before a predetermined date.

Roughly $10 billion in new shares are being contemplated to be sold by the company along with nearly $5 billion in mandatory convertible bonds, the sources said.

One of the four sources said the deal was scheduled to be priced shortly after Boeing's Oct. 23 third-quarter earnings report. But another investor source said the company was trying to avoid a raising during the month-old strike that analysts estimate is costing tens of millions of dollars per day.

"The timing of any equity raise is still unclear but market consensus is that it should be done after the labor strike is resolved and earnings provide some visibility of its impact on current and future cash flows," said Michael Barr, senior research analyst at Neuberger Berman.

While Boeing burned less free cash than expected during the third quarter, the planemaker may have no choice but to act before the end of the strike to protect its investment grade rating, two of the sources said.