The findings raise the possibility of automated "flying gas stations" topping off robotic aircraft over future battlefields.
The Central Intelligence Agency (CIA) regularly uses unmanned drones to target militants in countries like Pakistan, Afghanistan and Yemen.
The US Defence Advanced Research Projects Agency (DARPA) recently flew two modified RQ-4 Global Hawk drones in close proximity to simulate mid-air refuelling.
The Global Hawks, huge drones with 131-foot wingspans used for high-altitude surveillance, flew in formation with less than 100 feet separating the refuelling "probe" on one and refuelling "receiver" on the other during a two-and-a-half hour flight test.
"The goal of this demonstration was to create the expectation that future High Altitude Long Endurance aircraft will be refuelled in flight," Jim McCormick, program manager at DARPA said.
"Such designs should be more affordable to own and operate across a range of mission profiles than systems built to satisfy the most stressing case without refuelling.
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"The lessons from AHR certainly extend beyond the HALE flight regime, and insights into non-traditional tanker concepts may offer further operational advantages," McCormick said in a statement.
Neither Global Hawk drone needed human guidance during the final flight test at the high altitude of 44,800 feet. The drones also maintained their tight formation despite turns and wind gusts of up to 37 Kph.
DARPA had kicked off its two-year Autonomous High-Altitude Refuelling (AHR) program with the expectation that just one out of six aerial refuelling attempts would prove successful.
The final analysis, however, suggested drones could achieve a much better success rate of three out of five attempts.
The results from the AHR programme that ended on September 30 mean the US military has a good chance of extending the duration and range of its drone swarms, the statement said.
According to 'Tech News Daily', manned fighter jets and bombers already do mid-air refuelling from standard tanker aircraft, but many drones can't match the speed, altitude or performance of existing tanker aircraft.
McCormick suggested that the drone flight-testing could lead to "non-traditional tanker concepts," perhaps a reference to automated drone tankers. Such an idea could work very well for the range of mid-size or large drones that depend on jet fuel.