There are no direct flights from the USA to Kathmandu. However, there are many airlines that offer connecting flights through major hubs in Asia, such as:
nNew York City (JFK): Air India, Qatar Airways, Turkish Airlines, Etihad Airways
nLos Angeles (LAX): Air India, Qatar Airways, Emirates, Singapore Airlines
nChicago (ORD): Air India, Turkish Airlines, Etihad Airways
nSan Francisco (SFO): Air India, Qatar Airways, Singapore Airlines
nHouston (IAH): Air India, Emirates
nAtlanta (ATL): Qatar Airways, Turkish Airlines
nThe flight time from the USA to Kathmandu will vary depending on the hub you transit through, but it will typically take around 19-21 hours.