When Actavis was preparing to buy Allergan, it gave Mizuho Financial Group Inc. a weekend to decide if it wanted to help come up with $46 billion in loans to back the deal, which had been code-named Project Fruit Basket.

Mizuho Chief Executive Officer Yasuhiro Sato had an answer by Monday morning: Yes. Four months later, Japan's second-largest bank by assets was in line to lead a $21 billion bond sale that refinanced the loans.

Its role in the offering helped catapult Mizuho to the 11th-biggest underwriter of investment-grade notes in the U.S. this year, up 10 spots since 2011. Mizuho, stung by losses in the U.S. during the global credit crisis, has repeatedly pointed to the Actavis deal as indicative of its aspiration to take a greater share of the lucrative market.