The nation’s largest banks made a killing in the mortgage business last year.

Earnings released in the past several days show mortgage activity accounting for about 15 percent of the profits at Wells Fargo ($40.66 -0.18%). JPMorgan Chase ($53.55 -0.56%), meanwhile, reported $418 million in profit from its mortgage banking business, compared with a loss of $269 million a year earlier. Even Bank of America ($13.19 -0.08%), which has been retreating from the mortgage market, posted a 41 percent spike in loan originations in the fourth quarter, to $22 billion.