HDFC Bank raised $1.75 billion through a dual-tranche dollar bond sale, the largest one-shot debt fundraising by an Indian bank, three merchant bankers said on Friday, as banks scramble to take advantage of a concessional window from the central bank. India's largest private lender on Thursday raised $1.25 billion through the sale of five-year papers, equivalent to the largest single-tranche issuance, and $500 million through three-year papers via its GIFT City branch, it said in a notice to stock exchanges.