The Federal Reserve said Thursday that all 31 big banks it subjected to a stress test have sufficient capital to absorb losses during a sharp and prolonged economic downturn.

It's the first time since the central bank started stress tests in 2009 that no firm fell below any of the main capital thresholds. The two biggest banks that operate in Minnesota — Wells Fargo & Co. and U.S. Bancorp — easily exceeded the main ratios the Fed uses.

The annual tests, using hypothetical scenarios that are not forecasts, are the cornerstone of the Fed's efforts to prevent a repeat of the 2008 financial crisis and to gauge the ability of banks to withstand economic turmoil.

The largest U.S.-based banks "continue to build their capital levels and to strengthen their ability to lend to households and businesses during a period marked by severe recession and financial market volatility," the Fed said in a statement Thursday.

The Fed uses the exams to prod lenders into building up capital buffers. Banks that don't meet a second round of tests released next week may face restrictions in buying back stock and paying dividends.

This year's results are being released as the Fed faces scrutiny from lawmakers critical of its supervision of the biggest banks. Fed Chairwoman Janet Yellen countered the criticism, saying this week the central bank works hard to avoid the trap of "regulatory capture," or the risk that bank examiners get too cozy with the firms they oversee.

Bank of America Corp. was the only bank among the six largest to improve in every capital measure from its performance in last year's test. Wells Fargo & Co. surpassed every minimum by at least 2 percentage points. Morgan Stanley's ratio in three capital measures fell in a severely adverse scenario to within 1 percentage point of the required minimum.

Loan-loss estimates for the 31 banks totaled $490 billion under a hypothetical severely adverse scenario, down from $501 billion for the 30 banks tested last year. The losses include a $102.7 billion hit to trading. JPMorgan Chase & Co. would suffer the most from trading losses, estimated at $23.6 billion.

The heaviest damage was to consumer lending, with 39 percent of projected losses from such activity as mortgages and credit cards.

The severely adverse scenario this year included deep recessions in the euro area, U.K. and Japan, and "below-trend" growth in developing Asia, according to guidelines the Fed released in October.

While the test released on Thursday gauges banks' strength, the firms aren't said to pass or fail, because the exam doesn't account for their plans to preserve or bolster capital. In the tests to be released Wednesday, the Fed will take into account the companies' requested capital distributions and other measures, requiring they leave banks above a minimum Tier 1 common ratio of 5 percent over nine quarters in harsh economic scenarios.

Fed officials have cautioned that even banks that exceeded all the thresholds in Thursday's test may still find their requests for higher payouts rejected next week if regulators find the quality of the planning is flawed.